Fixes django 1.6 dup_select_related

There is a problem using django_polymorphic with django/master due to the removal of the dup_select_related method from QuerySet in this commit: https://github.com/django/django/commit/389892aae595b86c4be28c43e3312d76a68a0173

The commit message sounds a little tentative so, who knows? In the meantime this takes care of the problem.