tox: fix django 1.11 polymorphic dep

Author: fdintino

tox.ini

@@ -24,7 +24,8 @@ deps =
     django-selenosis
     dj-database-url
     python-monkey-business
-    django-polymorphic
+    dj111: django-polymorphic<3.0.0
+    dj{22,30,31}: django-polymorphic
     boto3==1.12.18
     django-storages==1.9.1
     Pillow