Remove unsupported python versions

Author: Devin McGloin

.travis.yml

@@ -1,10 +1,6 @@
 language: python
 python:
-  - "2.6"
   - "2.7"
-  - "3.3"
-  - "3.4"
-  - "3.5"
   - "3.6"
   - "3.6-dev"  # 3.6 development branch
   - "3.7-dev"  # 3.7 development branch