Updated User model reference based on Django recommendations.

Changed waffle.compat definition of User object to use
settings.AUTH_USER_MODEL instead of django.contrib.auth.get_user_model().
According to
https://docs.djangoproject.com/en/1.5/topics/auth/customizing/#django.contrib.auth.get_user_model
settings.AUTH_USER_MODEL should be used to define ForeignKey and
ManyToMany relationships instead of the actual User model from
get_user_model().