Update tox to use Django 2.2 final. (#6556)

Author: carltongibson

tox.ini

@@ -25,7 +25,7 @@ deps =
         django111: Django>=1.11,<2.0
         django20: Django>=2.0,<2.1
         django21: Django>=2.1,<2.2
-        django22: Django>=2.2rc1,<3.0
+        django22: Django>=2.2,<3.0
         djangomaster: https://github.com/django/django/archive/master.tar.gz
         -rrequirements/requirements-testing.txt
         -rrequirements/requirements-optionals.txt