install_package "yaml-0.1.4" "http://pyyaml.org/download/libyaml/yaml-0.1.4.tar.gz#36c852831d02cf90508c29852361d01b" --if needs_yaml
install_package "ruby-1.9.3-p194" "http://cache.ruby-lang.org/pub/ruby/1.9/ruby-1.9.3-p194.tar.gz#bc0c715c69da4d1d8bd57069c19f6c0e"