test_addrconv: Fix a way to import the tested module

I'm not quite sure how it happens to work with the current
test environment.  But the current coding doesn't work when
running via testr probably due to different PYTHONPATH.
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