Tox: update Django 2.1 tests to allow released version only

(and don't allow failures on Django 2.1 tests)

tox.ini

@@ -2,28 +2,30 @@
 envlist =
     # Test these environments first, to catch most errors early...
     lint
-    django20-py36
+    django21-py36
     django111-py27
     docs
     # ... then test all the other supported combinations:
+    django21-py{35,37,py3}
     django20-py{35,py3}
     django111-py{34,35,36,py2}
     # ... then prereleases (if available):
-    django21-py{35,36,37}
+    # django22-py{35,36,37}
     djangoMaster-py{36,37}
 
 [testenv]
 deps =
     django111: django~=1.11.0
     django20: django~=2.0.0
-    django21: django>=2.1a1
+    django21: django~=2.1.0
+    django22: django>=2.2a1
     djangoMaster: https://github.com/django/django/tarball/master
     # testing dependencies (duplicates setup.py tests_require):
     mock
     boto3
     sparkpost
 ignore_outcome =
-    django21: True
+    django22: True
     djangoMaster: True
 usedevelop = True
 args_are_paths = False
@@ -88,6 +90,7 @@ DJANGO =
     1.11: django111
     2.0: django20
     2.1: django21
+    2.2: django22
     master: djangoMaster
 LINT_AND_DOCS =
     true: lint, docs