travis should run tests in all environment combinations

[willkg]

Travis should run the tests in all the environment combinations. Plus it'd be nice to have a way to make sure tox.ini and .travis.yml are in sync.

[erikrose]

Worst case, it could run a pared-down tox config that would take care of different pip versions, while Travis itself took care of Python versions. That's essentially what http://jsatt.com/blog/using-tox-with-travis-ci-to-test-django-apps/ does.

[erikrose]

This should solve all our problems: https://github.com/msabramo/python-panci. We don't even necessarily have to use it; following the pattern of its multi-python-version example travis configs might be enough: it looks like they set up a single travis env that has multiple versions of Python installed and then simply run tox in it.