travis: Run tests via tox

Also, run pep8 in a separate environment.

The way to specify python versions might seem awkward.
(Well, it seems so to me.)
But it follows one of examples referenced in the official
documentation:
    http://docs.travis-ci.com/user/languages/python/
    https://github.com/twisted/klein/blob/master/.travis.yml
travis: Install tox