Add environment markers to django test dependency (#460)

diff --git a/tox.ini b/tox.ini
index 7a3bad2..444213e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,7 +13,8 @@
        mox
        pyopenssl
        pycrypto==2.6
-       django
+       django<2.0.0; python_version < '3.0.0'
+       django>=2.0.0; python_version > '3.0.0'
        webtest
        nose
        coverage>=3.6,<3.99