Travis builds fail due to CMake version

[goranbrkic]

CMakeLists file declares that minimum supported is 3.8, but on Travis it seems that osx_image: xcode8 has version 3.6.1.
On develop branch xcode8 is not part of the test build, so it makes sense to remove it for pull requests as well.

This is the cause why following PRs failed:

• Added new method in Crow class: #41
• Fixed problem with segmentation fault #43
• Fix issue with cmake on windows system outside msbuild environment #44

[nlohmann]

We use homebrew to update CMake on macOS builds. However, this seems to fail as sphinx-doc cannot be installed:

==> Installing cmake dependency: sphinx-doc
2464Warning: Your Xcode (8.0) is outdated.
2465Please update to Xcode 8.2.1 (or delete it).
2466Xcode can be updated from the App Store.
2467
2468Warning: A newer Command Line Tools release is available.
2469Update them from Software Update in the App Store.
2470
2471==> Downloading https://files.pythonhosted.org/packages/f6/3a/c51fc285c0c5c30bcd
2472######################################################################## 100.0%
2473==> Downloading https://files.pythonhosted.org/packages/11/74/2c151a13ef41ab9fb4
2474######################################################################## 100.0%
2475==> python3 -c import setuptools... --no-user-cfg install --prefix=/private/tmp/
2476==> python3 -s /private/tmp/sphinx-doc--homebrew-virtualenv-20191121-21416-pfjdc
2477==> Downloading https://files.pythonhosted.org/packages/cc/b4/ed8dcb0d67d5cfb7f8
2478######################################################################## 100.0%
2479==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2480==> Downloading https://files.pythonhosted.org/packages/bd/78/9fb975cbb3f4b136de
2481######################################################################## 100.0%
2482==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2483==> Downloading https://files.pythonhosted.org/packages/06/b8/d1ea38513c22e8c906
2484######################################################################## 100.0%
2485==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2486==> Downloading https://files.pythonhosted.org/packages/fc/bb/a5768c230f9ddb03ac
2487######################################################################## 100.0%
2488==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2489==> Downloading https://files.pythonhosted.org/packages/84/f4/5771e41fdf52aabebb
2490######################################################################## 100.0%
2491==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2492==> Downloading https://files.pythonhosted.org/packages/ad/13/eb56951b6f7950cadb
2493######################################################################## 100.0%
2494==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2495==> Downloading https://files.pythonhosted.org/packages/41/f5/3cf63735d54aa9974e
2496######################################################################## 100.0%
2497==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2498==> Downloading https://files.pythonhosted.org/packages/93/ea/d884a06f8c7f9b7afb
2499######################################################################## 100.0%
2500==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2501==> Downloading https://files.pythonhosted.org/packages/b9/2e/64db92e53b86efccfa
2502######################################################################## 100.0%
2503==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2504==> Downloading https://files.pythonhosted.org/packages/6a/f3/7cfe4c616e4b9fe055
2505######################################################################## 100.0%
2506==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2507==> Downloading https://files.pythonhosted.org/packages/16/51/d72654dbbaa4a4ffbf
2508######################################################################## 100.0%
2509==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2510==> Downloading https://files.pythonhosted.org/packages/7e/ae/26808275fc76bf2832
2511######################################################################## 100.0%
2512==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2513==> Downloading https://files.pythonhosted.org/packages/5d/3a/24d275393f493004ae
2514######################################################################## 100.0%
2515==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2516==> Downloading https://files.pythonhosted.org/packages/77/f9/8c63766fe271549db3
2517######################################################################## 100.0%
2518==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2519==> Downloading https://files.pythonhosted.org/packages/df/d5/3e3ff673e8f3096921
2520######################################################################## 100.0%
2521==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2522==> Downloading https://files.pythonhosted.org/packages/01/62/ddcf76d1d19885e857
2523######################################################################## 100.0%
2524==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2525==> Downloading https://files.pythonhosted.org/packages/dd/bf/4138e7bfb757de47d1
2526######################################################################## 100.0%
2527==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2528==> Downloading https://files.pythonhosted.org/packages/20/6b/d2a7cb176d4d664d94
2529######################################################################## 100.0%
2530==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2531==> Downloading https://files.pythonhosted.org/packages/1b/71/8bafa145e48131049d
2532######################################################################## 100.0%
2533==> /usr/local/Cellar/sphinx-doc/2.2.1/libexec/bin/pip install -v --no-deps --no
2534==> Downloading https://files.pythonhosted.org/packages/57/5f/bf9a0f7454df68a7a2
2535
2536curl: (6) Could not resolve host: files.pythonhosted.org
2537Error: An exception occurred within a child process:
2538  DownloadError: Failed to download resource "sphinx-doc--sphinxcontrib-devhelp"
2539Download failed: https://files.pythonhosted.org/packages/57/5f/bf9a0f7454df68a7a29033a5cf8d53d0797ae2e426b1b419e4622726ec7d/sphinxcontrib-devhelp-1.0.1.tar.gz
2540
2541cmake version 3.6.1

I need to look into this.

[nlohmann]

I removed the older Xcode builders. If Travis succeeds, I'll re-trigger the PRs.

[goranbrkic]

Should new PRs be branched out from develop? Or will these changes be merged back into master?