Synchronise travis and tox

See comment: https://github.com/spookylukey/django-wiki/commit/6f50923999ac6a5364f75de67dda1e8090180cda

Note that with these changes, the only thing that needs to be synced is the list of envs in tox.ini and .travis.yml, and there is a comment in tox.ini reminding you to do so.

This depends on my previous PR, you might want to pull that first.