Use AUTH_USER_MODEL from project settings

Referencing this: https://groups.google.com/forum/#!msg/django-developers/lmT1JxsWHPo/N2V9U5SN9gwJ

Was seeing this error: https://gist.github.com/stickwithjosh/5bbd4d6387266e8f2d32

This sorts it out. Hooray!