use `get_queryset` for Django 1.6+

Django changed from `get_query_set()` to `get_queryset()` in Django 1.6 (which raises deprecation warnings).
Switched to this method. Removing `get_query_set` for Django 1.6+ does break BC. Though we could get away with it by saying it only takes effect when upgrading Django versions *and* since this component doesn't technically support 1.6 yet. But this is truer to SemVer and will throw a warning only for 1.6+. When ArmSections 2.0 hits we can go back to:

    if django.VERSION < (1, 6):  # DROPWITHDJANGO15
        get_query_set = get_queryset