Add additional python versions to unit testing

 - Add Python 3.5 and 3.6 to tested versions in tox.ini
 - Add individual python versions to .travis.yml to take advantage of
   Travis-CI's parallel testing
 - Bump py.test version to support Python 3.5 and 3.6 testing
 - Migrate from Travis-CI's legacy infrastructure to container infrastructure
   by adding `sudo: false` to .travis.yml
 - virtualenv >= 14.0.0 removed support for python 3.2. Add conditional
   installation of virtualenv and py.test for 3.2 environments
3 files changed