Tests fail on Django 1.6

https://travis-ci.org/fusionbox/django-authtools/jobs/12636076#L186

This is due to https://github.com/django/django/commit/4a9bae0b39aebdedf38fcb760405c04b8216c508 and https://code.djangoproject.com/ticket/21164 and how we reuse Django's tests.