Update import for `GenericRelation`
Update import for `GenericRelation`

As of Django 1.7 the GenericRelation has been moved from
django.contrib.contenttypes.generic to
django.contrib.contenttypes.fields. This is an update to the ReadMe to
reflect this change.

bitmazk/django-user-media,bitmazk/django-user-media,bitmazk/django-user-media