Make tox dependencies more flexible

Author: knaperek

tox.ini

@@ -6,7 +6,7 @@ commands =
     python tests/run_tests.py
 
 deps =
-    django18: Django~=1.8.15
-    django19: Django~=1.9.10
-    django110: Django~=1.10.2
+    django18: Django>=1.8,<1.9
+    django19: Django>=1.9,<1.10
+    django110: Django>=1.10,<1.11
     djangomaster: https://github.com/django/django/archive/master.tar.gz