Drop support for Python 3.3 (robotframework#1143)

Fixes robotframework#1141

Author: aaltat

.travis.yml

@@ -62,7 +62,7 @@ matrix:
  - ROBOTFRAMEWORK=2.8.7
  - ROBOT_OPTIONS=
  - INTERPRETER=python2
- - python: "3.3"
+ - python: "3.4"
  env:
  - BROWSER=chrome
  - SELENIUM=2.53.6