Use get_queryset instead of get_query_set

Starting in Django 1.8, get_query_set will now be get_queryset. This change was
announced as part of the Django 1.6 release and now in Django 1.7 throws a
RemovedInDjango18Warning. See:

https://docs.djangoproject.com/en/1.7/releases/1.6/#get-query-set-and-similar-methods-renamed-to-get-queryset