Run pyflakes on travis.

.travis.yml

@@ -18,9 +18,11 @@ matrix:
 install:
   - pip install -U pip
   - pip install tox
+  - pip install pyflakes
   - sudo rm -rf /dev/shm && sudo ln -s /run/shm /dev/shm
 script:
   - tox -e $TOX_ENV
+  - python -m pyflakes.__main__ $(git ls-files | grep -E "^isso/.+.py$")
 notifications:
   irc:
     channels: