django 1.6 support

In django 1.6.8 there is a "cant set attribute" error when setting `query_set` in `admin.py`, it seems like this is name is [deprecated](https://docs.djangoproject.com/en/1.6/releases/1.6/#get-query-set-and-similar-methods-renamed-to-get-queryset). This PR tries to address that issue.