In CI, run style checks before unit tests

[capelastegui]

Some CI builds pass all tests (which take several minutes), and then fail due to some code style issues. It would probably be better to have the style checks run first.

In travis.yml, change this:

script:
  - coverage run -m unittest discover
  - pycodestyle ./ --ignore=E402,W

to this:

script:
  - pycodestyle ./ --ignore=E402,W
  - coverage run -m unittest discover