User get_absolute_url fixes

The proxy class approach used to provide get_absolute_url for the user class in the initial port to Django 1.7 doesn't work correctly for a couple of our uses, leading to things like the "profile" button on the navigation menu not working correctly.

With the release of Django 1.7.1, it's possible to use ABSOLUTE_URL_OVERRIDES to do the right thing. This also removes the need for the proxy class.

The .travis.yml tweak is an inelegant way of forcing the installation of the latest appropriate Django version, so I'm open to better suggestions.