Drop Python 3.3 and add Python 3.6 support

.travis.yml

@@ -2,9 +2,9 @@ language: python
 sudo: false
 python:
   - "2.7"
-  - "3.3"
   - "3.4"
   - "3.5"
+  - "3.6"
 # command to install dependencies
 install:
   - "pip install -r requirements-dev.txt"