bump ruby-build version to 20140919 for ruby 2.1.3 definintions
Merge pull request #117 from boxen/ruby-build-20140919

Bump ruby-build version to 20140919
Make explicit that ruby-build version should be managed on users repository

See https://github.com/boxen/puppet-ruby/pull/117#issuecomment-60547454

Closes #121