Update drf to latest version.

Changelog can be found here: http://www.django-rest-framework.org/topics/release-notes/#31x-series

requirements/base.txt

@@ -8,7 +8,7 @@ django-model-utils==1.5.0
 django-oscar==1.0.2
 django_simple_history==1.6.1
 django-waffle==0.10
-djangorestframework==3.1.0
+djangorestframework==3.1.2
 djangorestframework-jwt==1.5.0
 edx-auth-backends==0.1.3
 jsonfield==1.0.3