fix user_install recipe allowing specifying a hash for a ruby install

Fix `recipe[rbenv::user]` recipe for use in scenario below:

```
node.default['rbenv']['user_rubies'] = [ "1.8.7-p352",
  {
  :name => '1.9.3-327',
  :environment => { 'CFLAGS' => '-march=native -O2 -pipe' }
  }
]
```