Fix `add_media` util for Django 2.0

Neither `add_css` nor `add_js` exist in Django 2.0 because the method
for adding `Media` classes together has changed.

Ref: c19b56f633

polymorphic/formsets/utils.py

@@ -1,11 +1,17 @@
 """
 Internal utils
 """
+import django
 
 
 def add_media(dest, media):
     """
     Optimized version of django.forms.Media.__add__() that doesn't create new objects.
+
+    Only required for Django < 2.0
     """
+    if django.VERSION >= (2, 0):
+        dest += media
+    else:
         dest.add_css(media._css)
         dest.add_js(media._js)