Run a command against a specified set of rubies
$ mkdir -p ~/.rbenv/plugins
$ cd ~/.rbenv/plugins
$ git clone https://github.com/rodreegez/rbenv-only.git
rbenv only -r 1.9.2-p290,1.9.3-p0 -c "bundle exec rake"
This was mostly ripped off from rbenv-each.