Fix tests for Django 1.7

[beaugunderson]

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.

[beaugunderson]

Closing in favor of #93.