Use TravisCI's container infrastructure

This makes the startup time for tests much faster.

This also fixes an issue where bundle install would fail because the
version on travis's legacy infrastructure is too old.

See: https://github.com/bundler/bundler/issues/3558

.travis.yml

+sudo: false
 language: ruby
 language: ruby
 rvm:
 rvm:
   - 2.0.0
   - 2.0.0