Adding failing test case for django-mptt/django-mptt#316
Add backwards compatibility for django<1.7 custom Manager using custom QuerySet - fix django-mptt/django-mptt#316
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Update tox environments
Merge remote-tracking branch 'django-wiki/master'
Fix get_queryset/get_query_set horribleness.

Builds on @benjaoming's excellent PR (#317), with some minor renaming
and a backwards compatibility fix for get_queryset() support in
django 1.5.

(Fixes #316, #317)