Fix macOS test to install zlib explicitly

Author: Kroisse

.travis.yml

@@ -22,6 +22,7 @@ install:
     pip install --upgrade pip setuptools tox-travis
   elif [[ "$TRAVIS_OS_NAME" = "osx" ]]; then
     brew tap drolando/homebrew-deadsnakes
+    brew install zlib
     brew install python34 python35 python36 python3 pypy
     pip install --upgrade pip setuptools
     pip install tox