Updated travis and tox

[raiderrobert]

• Simplified tox.ini and .travis.yml configs significantly
• Added Django 2.0-master and pypy3

[EnTeQuAk]

Thanks a lot for the pull request and adding Django 2.0-master and pypy3 support.

I'm not sure I like the simplification though since this removes granularity from our Travis CI setup and removes the ability to follow-up on failures for a specific Django version. Since most failures occur as a result of changes in a Django version rather than incompatibility with Python.

Do you have the time to maybe rollback those changes so that we keep our granularity in Travis CI? Tox really is mostly being used to unify our test-execution while the "what we test" (python version, django version) is controlled via Travis.

[raiderrobert]

@EnTeQuAk I'd rather not completely roll these back, because the original config is overly complex. I'll take another swing at both simplifying while maintaining the discrete discovery you're after.

[raiderrobert]

@EnTeQuAk OK, done with my changes. Please re-review at your leisure.

[EnTeQuAk]

That's much better, and probably even faster and easier to cache.

Thanks a lot!