Update django-rest-framework to 3.7

In particular, this resolves the template error wrt crispyforms
when deploying.

requirements.txt

@@ -7,7 +7,7 @@ django-template-check>=0.3.0  # This should be in dev-requirements somehow
 bleach>=2.0.0,<2.1
 django-tinymce4-lite>=1.5,<1.6
 rcssmin>=1.0.6
-djangorestframework>=3.6.2,<3.7
+djangorestframework>=3.7,<3.8
 django-ical>=1.4,<2
 django-libsass>=0.7,<1
 django-cors-headers>=2.0.0,<2.1