Allow specifying custom rubies path

@Atalanta and @LordCope for review

# Problem

For people using custom ruby paths there is no way to specify a specific directory to look for rubies in.

# Solution

Using `node['chruby']['user_rubies']` array for custom paths. This attribute is optional and defaults to `[]`.

## Usage

Simply add the array to your role attributes.

For example:

```
default_attributes({
  'chruby' => {
    ,,,
    ,,,
    'user_rubies': [ '/usr/lib/custom_path' ]
  }
})
```

I've updated the attributes list in the README to indicate this.