Add workaround to support django 1.5

JSON serialization is different in django 1.5, and since django-roughage was
written to work with django 1.4, but should support django 1.5,
a serialization workaround was required.

This change does not change anything but the serializer.
Merge pull request #6 from Yipit/django-1.5-workaround

Add workaround to support django 1.5