Remove spurious super on an old stlye class errors on Models

Ever since I started using pylint-django, I started getting these error message whenever using super(DerivedModel, self).anything() on classes that derive from the Django model class.

apps/accts/models/invitation.py:116: [E1002(super-on-old-class), Invitation.save] Use of super on an old style class

This pull request fixes this.