Remove unsupported Python versions from TravisCI.

Keep Python 2.7 around, although not supported I can't pretend it's not
in use.

.travis.yml

 ---
 language: python
-python: ["2.7", "3.3", "3.4", "3.5", "3.6"]
+python: ["2.7", "3.6"]
 dist: trusty
 sudo: false
 group: beta