Enable grape-rabl to render correct rabl version of the object

Add the version to the template file path so that the correct version of the rabl is rendered.

```ruby
class API::V1::Users < Grape::API
  version 'v1', using: :header, vendor: 'leeo'
  format :json
  formatter :json, Grape::Formatter::Rabl
end

class API::V2::Users < Grape::API
  version 'v2', using: :header, vendor: 'leeo'
  format :json
  formatter :json, Grape::Formatter::Rabl
end

class Application < Rails::Application
    config.middleware.use(Rack::Config) do |env|
        env['api.tilt.root'] = Rails.root.join "app", "views", "api"
    end
end
```

With that setup, the template path will now contain the env['api.version']. If there isn't a version specified, then the original path will work fine.

I think the specs will have to include these use cases of versioning.