add 3.5 and 3.6 versions of python to travis ci

.travis.yml

@@ -1,6 +1,9 @@
 sudo: false
 language: python
-python: "3.4"
+python:
+  - "3.4"
+  - "3.5"
+  - "3.6"
 
 env:
   - DB=postgres