Use the old django test runner

We aren't ready to switch to the new unittest discovery in django.
ProxyGenericForeignKey should use for_concrete_model when possible
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
More efficient travis installation
Reorder tests to put more interesting ones first
install with pip

setup.py will fail with dependency issues, pip will fix them. After pip
'fixes' them, we can really fix them. This allows us to override the
django < 1.6 requirement in Mezzanine.
Pin django_polymorphic to the version that supports django 1.6