Add short comment for travis flake8

.travis.yml

@@ -38,4 +38,5 @@ script:
     - cd ..
     - for f in doc/examples/*.py; do $PYTHON "$f"; if [ $? -ne 0 ]; then exit 1; fi done
     - for f in doc/examples/applications/*.py; do $PYTHON "$f"; if [ $? -ne 0 ]; then exit 1; fi done
+    # Run pep8 and flake tests
     - flake8 --exit-zero --exclude=test_*,six.py skimage doc/examples viewer_examples