Add python 3.6 to build matrix

Author: Ryan P Kilby

.travis.yml

@@ -7,6 +7,7 @@ python:
   - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
 env:
   - DJANGO="django>=1.10.0,<1.11.0" REST_FRAMEWORK="djangorestframework>=3.5,<3.6"
   - DJANGO="django>=1.9.0,<1.10.0" REST_FRAMEWORK="djangorestframework>=3.5,<3.6"