Add python 3.6 to travis testing matrix, change postgres from 9.4 to 9.6

.travis.yml

@@ -2,6 +2,7 @@ language: python
 python:
   - "2.7"
   - "3.5"
+  - "3.6"
 install:
     - pip install -r requirements.txt
     - pip install braintree coveralls mock mysqlclient reportlab
@@ -10,7 +11,7 @@ env:
   - DB=mysql
   - DB=postgres
 addons:
-  postgresql: "9.4"
+  postgresql: "9.6"
 script:
   - coverage run --omit=*/tests/* --source=hc manage.py test
 after_success: coveralls