Update test matrix

This pull request makes the following updates to `tox.ini` and `.travis.yml`:

1. Use South 0.8.1 instead of 0.7.6.

2. Since South 0.8 supports Python 3, install South for the Python 3 tests.

3. Test against both the stable Django 1.6 release branch and Django development master.

4. Don't run tests on Python 2.6 with Django development master, which now supports 2.7+ only.

5. No longer use `--use-mirrors` for installation of dependencies; the public PyPI mirror network is deprecated now that PyPI itself runs off a CDN.

@treyhunner mind reviewing this before I merge it?