Use get_queryset() instead of get_query_set()

The latter is deprecated in Django 1.7.
Alias get_query_set to get_queryset

As per Django 1.6 release notes:
https://docs.djangoproject.com/en/dev/releases/1.6/#get-query-set-and-similar-methods-renamed-to-get-queryset
get_query_set -> get_queryset in docs for consistency
Merge pull request #80 from ebrelsford/master

Use get_queryset() instead of get_query_set()