Django 1.7

Two small changes to make django-nose work with django 1.7 and 1.8.
* In Django >= 1.7, some setup is required before running tests
* In Django 1.8, `django.test.simple.DjangoTestSuiteRunner` has been removed in favor of `django.test.runner.DiscoverRunner`

Caveats:
* I did not test with the new built-in migrations in Django 1.7. I have, however, tested basic test running in a django 1.7 app.
* I disabled tests with south because South doesn't and won't support Django >= 1.7.
* I turned off pip mirrors in `.travis.yml` because it was breaking travis builds, which are passing on the PR. If you'd like me to revert that change, I will.