Temporarily pinned Django versions

There appears to be a bug in Django 1.9.1/1.8.7 causing the migrations to fail.

tox.ini

@@ -39,8 +39,8 @@ deps =
     coverage
 
     dj17: Django>=1.7.1,<1.8
-    dj18: Django>=1.8,<1.9
-    dj19: Django>=1.9,<1.10
+    dj18: Django==1.8.7 # Temporily pinned to work around bug in Django 1.8.8
+    dj19: Django==1.9.0 # Temporily pinned to work around bug in Django 1.9.1
     postgres: psycopg2>=2.6
     mysql: mysqlclient==1.3.6