Add Django version (#162)

tox.ini

@@ -1,9 +1,22 @@
 [tox]
-envlist=py27,py34,py35,flake8
+# sort by django version, next by python version
+envlist=
+    flake8
+    py{27,34}-django17
+    py{34,35}-django18
+    py{27,34,35}-django19
+    py{27,34,35}-django110
+    py{27,34,35,36}-django111
+    py{34,35,36}-django20
 
 [testenv]
 deps=
-    django
+    django17: django>=1.7,<1.8
+    django18: django>=1.8,<1.9
+    django19: django>=1.9,<1.10
+    django110: django>=1.10,<1.11
+    django111: django>=1.11,<2.0
+    django20: django>=2.0,<2.1
     -r{toxinidir}/test_requirements.txt
 sitepackages=False
 commands=