Use the old django test runner

We aren't ready to switch to the new unittest discovery in django.
ProxyGenericForeignKey works on Django 1.6 now

Our code was broken by ForeignObject.
import urls from the non-deprecated place
ct.model_class() behavior is different for nonexistent models

in django < 1.6 it raises AttributeError, in 1.6 it returns None
Rename field_names to get_formfield_labels

You can't have a queryset attribute called field_names in django 1.6
Add django master to travis config