fix definition when the rubie is a Hash

fix definition when the rubie is a Hash

### Example


```ruby
:rbenv => {
  :user_installs => [
    {
      :user   => "vagrant",
      :rubies => [
        {
          :name => '2.1.0',
          :environment => { 'CFLAGS' => '--disable-install-doc --enable-shared' }
        }
      ],
      :global => "2.1.0",
      'gems'  => {
        '2.1.0'    => [
          { 'name'    => 'bundler' }
        ]
      }
    }
  ]
}
```

Error
`Option definition must be a kind of String!  You passed {"name"=>"2.1.0"}.`