Merge pull request #19 from edx/will/upgrade-django-model-utils

Upgrade django-model-utils to 2.3.1

requirements.txt

 dogapi==1.2.1
 django>=1.4,<1.5
 django-extensions==1.5.5
-django-model-utils==1.4.0
+django-model-utils==2.3.1
 djangorestframework<2.4
 pytz==2015.2
 South==0.7.6