fix behavior when ruby versions passed as hashes instead of string

this fixes the handling of rubies when passed as hashes, i.e.

```ruby
node['rbenv'] => {
  'user_rubies' => [
    { 'name' => 'ree-1.8.7-2012.02', 'environment' => { 'CONFIGURE_OPTS' => '--no-tcmalloc' }  }
  ]
}
```

and also the name passed to the rbenv_ruby lwrp whether it's a hash or a string.