Ensuring ::Rails::Engine is referenced

Hi Tim,

I ran across an issue with the [Rails::Engine reference](https://github.com/tmcgilchrist/roy-rails/blob/bc57d7247689a646f26273345ccffde67afd90ec/lib/roy/roy_rails.rb#L2) with the gem installed on Rails v3.2.11, it raised an

```ruby
uninitialized constant Roy::Rails::Engine (NameError)
```

The included change just uses `::Rails::Engine` instead of `Rails::Engine`

Cheers,
 -Eric