Remove dependencies on django-nose and nose

This removes the dependency on django-nose, which is one of the things blocking Django 1.7 compatibility.

I ended up also removing the dependency on nose, after spending some time trying to update `run_tests.py` to properly use nose as the test runner for Django tests across four different versions of Django. I can see from that the benefit of using django-nose, if you need to use nose, but I don't see any great advantage to running rapidsms tests with nose and I like removing dependencies when we can.

Finally, this fixes a couple of bugs in the tox.ini config file that were resulting in running the tests against 1.6 alpha instead of 1.6.latest, and some other old version (I forget exactly) instead of Django trunk.

This does not get tests passing against 1.7 or trunk; we have another pull request already working on that. But this should help with that.