It parses the result of `rbenv versions`:

```
  system
  1.9.3-p448
  2.0.0-p247
  2.0.0-p353
* 2.1.0 (set by /Users/bodo/code/dotfiles/.ruby-version)
  2.1.0-rc1
  jruby-1.7.4
  jruby-1.7.9
```

The regexp in rvm is sadly not better.