update .travis python version

Author: ZhangBohan

.travis.yml

@@ -3,8 +3,7 @@ python:
   - "2.7"
   - "3.4"
   - "3.5"
-  - "3.5-dev" # 3.5 development branch
-  - "3.6-dev" # 3.6 development branch
+  - "3.6"
   - "nightly" # currently points to 3.7-dev
 install:
  - pip install -q Django==$DJANGO_VERSION