Test against Django 2.1 final

tox.ini

@@ -43,7 +43,7 @@ deps =
     dj111: Django>=1.11b1,<2.0
     dj111-mssql: django-pyodbc-azure==1.11.0.0
     dj20: Django>=2.0,<2.1
-    dj21: git+https://github.com/django/django.git@2.1rc1#egg=Django
+    dj21: Django>=2.1,<2.2
 
     postgres: psycopg2>=2.6
     mysql: mysqlclient==1.3.6