use "rbenv root" to determine a path to available ruby versions

It fixes https://github.com/grosser/wwtd/issues/8.

`rbenv root` is used (instead of `which rbenv`) to determine a path to the ruby version.

From rbenv documentation: "rbenv root command prints the value of $RBENV_ROOT, or the default root directory if it's unset."

```shell
$ rbenv root
/Users/lukkry/.rbenv

$ ls `rbenv root`/versions
1.9.3-p392	1.9.3-p448	2.0.0-p247

$ rbenv versions
  system
  1.9.3-p392
  1.9.3-p448
* 2.0.0-p247 (set by /Users/lukkry/projects/zendesk/zendesk_voice_core/.ruby-version)
```