Updated url template tag usage to be Django 1.5 compatible

I couldn't see any information relating to what Django versions you are supporting with Nexus.

This breaks compatibility with Django 1.2.x (assuming that it was compatible in the first place).

This change *should* be compatible with Django >= 1.3 -- there were no tests to run to make sure! It was working for me ok in a virtualenv with Django 1.3.1 and Django 1.5 alpha.