Add support for Django 2.2 (by testing it in the CI)

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py27-django111
-    py{35,36,37}-django{111,20,21}
+    py{35,36,37}-django{111,20,21,22}
     lint
 
 [testenv]
@@ -11,7 +11,8 @@ pip_pre = true
 deps =
     django111: django>=1.11,<2.0
     django20: django>=2.0,<2.1
-    django21: django>=2.1
+    django21: django>=2.1,<2.2
+    django22: django>=2.2
     appdirs==1.4.3
     py27: mock==2.0.0