Upgrade django-model-utils, make tox Python version explicit

requirements.txt

 dogapi>=1.2.1,<2.0.0
-django-model-utils>=2.3.1,<3.0.0
+django-model-utils>=2.3.1
 # Use the same DRF version as edx-platform
 git+https://github.com/edx/django-rest-framework.git@1ceda7c086fddffd1c440cc86856441bbf0bd9cb#egg=djangorestframework==3.6.3
 jsonfield>=2.0.2,<3.0.0

tox.ini

 [tox]
-envlist = django{18,110,111}
+envlist = py27-django{18,110,111}
 
 [testenv]
 deps =