travis.yml: run flake8 tests before test suite
travis.yml: run flake8 tests before test suite

Since unit tests take longer to run than flake8 tests, run the latter
first so builds abort sooner if necessary.

Signed-off-by: Niklas Koep <342d5290239d9c5264c8f98185afedb99596601a@gmail.com>

pymanopt/pymanopt,nkoep/pymanopt,nkoep/pymanopt,nkoep/pymanopt,pymanopt/pymanopt