Add possibility to select Ruby by version only, without a patch

It will be nice if there will be possible to select Ruby in more open-ended way. Like if I write `rbenv shell 2.0.0` it will select the newest installed Ruby version with selected prefix. There is [`rbenv-use`](https://github.com/rkh/rbenv-use) but it try to use last published Ruby with given prefix, not the newest installed.

In my [fish rbenv implementation](https://gist.github.com/hauleth/5337768) I have created function `rbenv_lookup` that find the newest installed Ruby with given prefix.

```
function rbenv_lookup
  set -l vers (command rbenv versions --bare | sort | grep -- "$argv[1]" | tail -n1)
 
  if [ ! -z "$vers" ]
    echo $vers
    return
  else
    echo $argv
    return
  end
end
```

I know that it's not ideal but work nicely, even if I write only `rbenv shell 2` it will change to the newest installed Ruby 2.