Upgrade django-model-utils to 2.3.1

requirements/base.txt

@@ -6,7 +6,7 @@
 git+https://github.com/edx/XBlock.git@9c634481dfc85a17dcb3351ca232d7098a38e10e#egg=XBlock
 
 # edx-submissions
-git+https://github.com/edx/edx-submissions.git@906c7b7a27e1a8684834334a4bf7fb50a3aede30#egg=edx-submissions
+git+https://github.com/edx/edx-submissions.git@9538ee8a971d04dc1cb05e88f6aa0c36b224455c#egg=edx-submissions
 
 # Third Party Requirements
 boto>=2.32.1,<3.0.0
@@ -15,7 +15,7 @@ defusedxml==0.4.1
 django>=1.4,<1.5
 django-celery==3.1.16
 django-extensions==1.5.5
-django-model-utils==1.4.0
+django-model-utils==2.3.1
 djangorestframework<2.4
 dogapi==1.2.1
 lazy==1.1