Change django.conf.urls.defaults -> django.conf.urls

Django 1.4 changed the namespace for 'patterns' and 'url' from
django.conf.urls.defaults to django.conf.urls. This was listed in the
deprecation notes, but we never updated it because 1.4 maintained
compatibility. 1.5 got rid of the compatibility shim, so we need to actually
update now.

Testing done: Ran unit tests

Reviewed at https://reviews.reviewboard.org/r/4566/