Add 'bob --help' to the Travis run

In lieu of having a test suite, this at least ensures the setup.py
`console_scripts` entry is configured correctly, and that basic
CLI and arg functionality works.

Author: edmorley

.travis.yml

@@ -7,6 +7,10 @@ python:
   - "3.5"
   - "3.6"
 install:
+  - pip install .
   - pip install -r requirements.txt
 script:
   - flake8
+  # TODO: Replace with an actual test suite:
+  # https://github.com/kennethreitz/bob-builder/issues/31
+  - bob --help