Add Django 1.4 to Travis testing.
Work around get_user_model().

* Django 1.4 doesn't have custom user models, so the
  get_user_model() method doesn't exist, and we need
  to default to django.contrib.auth.models.User.
Correct syntax on the Travis exclusions.
Clean up the Django <1.5 compatibility code.
Merge pull request #7 from incuna/i7-support-django-1.4

Support Django 1.4(.13)