store buildpack ruby_version data
install Ruby 1.9.3 by default
install last version of ruby used
don't require cache for using metadata
update comment
set ruby_version_env_var properly
fix new_app? detection code
use BUNDLE_VERSION so we know we're using the right bundler version
fallback on ruby 1.9.2 if it's not a new app
warn when ruby version is not set in the Gemfile