Ok, I've created `~/bin/jruby` which does this (for future posterity):

    #!/bin/sh

    RBENV_VERSION=`rbenv versions | grep jruby | tail -1 | perl -pe 's/^.*(jruby-[^\s]+).*$/$1/'` ~/.rbenv/shims/jruby "$@"

Will change this PR into a README update.