Travis now runs flake8 on pull requests

This will cause flake8 to run on code introduced by the current commit. This should work even on pull requests with multiple commits because Travis runs the tests against the merge commit between the origin and upstream branches[0].

[0] http://docs.travis-ci.com/user/pull-requests/#How-Pull-Requests-are-Tested
There is already a flake8.cfg file in the project root, but only in the master branch. It might be best to copy that file into this branch so we don't have both.