Drop support for Django 1.7 and below

[clintonb]

Django 1.8 is the current long-term support version of Django. There is some code that mentions Django 1.6; but, the South migrations haven't been update along with the standard Django migrations. This could probably fixed; but, it would be simpler to drop support for older versions and remove the cruft from the codebase.

[omab]

I've created python-social-auth/social-app-django#5 to properly track this work.