Update tox matrix

Author: tsouvarev

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{35,36}-djangomaster, py{27,34,35,36}-django{18,111,20}, lint, docs
+envlist = py{35,36,37}-django{21,master}, py{27,34,35,36}-django{18,111,20}, lint, docs
 
 [testenv]
 deps =
@@ -11,6 +11,7 @@ deps =
     django18: Django>=1.8,<1.9
     django111: Django>=1.11,<2.0
     django20: Django>=2.0,<2.1
+    django21: Django>=2.1,<2.2
     djangomaster: https://github.com/django/django/archive/master.tar.gz#egg=Django
 commands = py.test {posargs}