Test against Django 1.4.6 & 1.5.2 (updated from 1.4.5 & 1.5.1)

Opening this for discussion due to the recent [security releases](https://www.djangoproject.com/weblog/2013/aug/13/security-releases-issued/). The 1.3.x branch received no updates.

Apologies for the crap branch name; it's the automatic one applied when you edit via github.

Tox tests passed locally on all interpreters.