travis: Run pycodestyle on commits.

Run pycodestyle in continuous integration. This will cause
jobs to fail if new warnings are introduced, which helps
maintain a clean codebase where new issues are more obvious.

Ignore E501 line to long, since local style doesn't respect it.

.travis.yml

@@ -7,11 +7,12 @@ python:
 # command to install dependencies
 install:
     - pip install -r requirements.txt
-    - pip install coveralls
+    - pip install coveralls pycodestyle
 
 # command to run tests
 script:
     - coverage run setup.py test
+    - pycodestyle --ignore E501 *.py HookTest
 after_success:
     - coverage combine
     - coveralls