Fix tests for Django 1.7

This PR fixes tests for 1.7.

- The defaults for `MIDDLEWARE_CLASSES` changed so we need to specify sane defaults.
- There's something going in with model names in fixtures in 1.7 for sub-apps; `oauth2.client` becomes `provider.client` in the fixture.
- We need to specify the new test runner in 1.7 to avoid warnings.
- I added the 1.7 tests to `.travis.yml`.