chore(travis): run flake8 after tests

.travis.yml

@@ -23,7 +23,7 @@ install:
   - pip install ".[test]"
   # we need this branch to perform testing, do it after the pip install above, otherwise we 'downgrade'
   - pip install --upgrade --no-deps --force-reinstall git+https://github.com/jupyter/nbconvert.git@master#egg=nbconvert
-before_script:
+after_script:
   - flake8 voila
 script:
   - VOILA_TEST_DEBUG=1 py.test tests/