This gem is meant for Ruby 1.8.x builds. Ruby 1.9.x comes with a functioning ripper implementation out of the box. This gem is a port of the Ripper 1.9 for Ruby 1.8.x. Ripper was written by Minero Aoki and ported by Loren Segal.
Gemfile gem 'ripper', '~> 1.0.1'
install gem install ripper -v 1.0.1