Add flake8 to travis (see PR #87)

.travis.yml

@@ -32,6 +32,15 @@ install:
   - pip install coverage codecov pytest>=3.0.5 pytest-cov codacy-coverage # nose>=1.3.7
   - pip install .
 
+before_script:
+  - pip install flake8
+  # stop the build if there are Python syntax errors or undefined names
+  - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics
+  # exit-zero treats all errors as warnings.  The GitHub editor is 127 chars wide
+  # currently deactivated as style guidelines are not yet kept in the project
+  # TODO change this
+  #- flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics
+
 script:
   - python tests/test.py
   - coverage run --source imgaug --omit ./imgaug -m pytest tests/test.py --verbose --doctest-modules