Travis: Flake8 only once and fail fast
Travis: Flake8 only once and fail fast

The flake8 command was failing but the rest of the build was still
running and marked as failure only after the entire test suite was run
through. This is a travis issue as can be seen
[here](https://github.com/travis-ci/travis-ci/issues/1066).

Also moved it to be ran before the travis matrix commands.

charles-cooper/raiden,charles-cooper/raiden,tomashaber/raiden,hackaugusto/raiden,tomashaber/raiden,tomashaber/raiden,tomashaber/raiden,tomashaber/raiden,hackaugusto/raiden