Travis: Install Django 1.7 from PIP
Removed support for Django 1.6 and Python 2.6
Deleted south migrations
Always use Pythons builtin unittest module

As we've dropped Python 2.6, the builtin unittest module will always be
unittest2
Cleaned up backend loading code
Removed Django 1.6 support from search query
Removed Django 1.6 support from utils/apps.py
Removed Django 1.6 support from test_migrations
Removed Django 1.6 support from test settings