Fixing django 1.6 support.
Fixing django 1.6 support.

fabrique/django-sortedm2m,fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,gradel/django-sortedm2m,gradel/django-sortedm2m,gregmuellegger/django-sortedm2m,fabrique/django-sortedm2m,gradel/django-sortedm2m,gregmuellegger/django-sortedm2m