Ensure that rest framework is compatible with django 2.0

3.7.4 is the first release with django 2.0 compatibility
http://www.django-rest-framework.org/topics/release-notes/#374

setup.py

@@ -25,7 +25,7 @@ install_requires = [
     "django-modelcluster>=4.0,<5.0",
     "django-taggit>=0.22.2,<1.0",
     "django-treebeard>=4.2.0,<5.0",
-    "djangorestframework>=3.1.3,<4.0",
+    "djangorestframework>=3.7.4,<4.0",
     "draftjs_exporter>=2.0,<2.1",
     "Pillow>=2.6.1,<5.0",
     "beautifulsoup4>=4.5.1,<5.0",