.travis.yml: Make it leverage tox to be DRY

This makes the `.travis.yml` leverage `tox.ini` so we have:

- less duplication (though there is not much)
- (more importantly) they are using the same mechanisms and get the same results -- I actually had a problem where the tox tests passed because they pinned pytest==2.5.2 but then the Travis tests failed because they don't go through the tox.ini.