Prefix `bundle exec` on `run_simple`

The test were passing fine locally since I had `rubygems-bundler` gem,
which setups the environment without needing to run `bundle exec`.
However, this does not work on TravisCI.
Turn off verbose for RSpec rake task
Update build matrix

* Install stable version of Bundler
* Add Ruby 2.1.0
* Change `rbx-19mode` to just `rbx`
* Allow failure on `rbx` :(