Un-pin Django Rest Framework <3.7. Reverses #3912

setup.py

@@ -25,7 +25,7 @@ install_requires = [
     "django-modelcluster>=3.1,<4.0",
     "django-taggit>=0.20,<1.0",
     "django-treebeard>=3.0,<5.0",
-    "djangorestframework>=3.1.3,<3.7",  # 3.7 drops Django 1.8 support
+    "djangorestframework>=3.1.3,<4.0",
     "Pillow>=2.6.1",
     "beautifulsoup4>=4.5.1",
     "html5lib>=0.999,<1",