add constant for Jquery UI version (fix #41)

This is useful so that you can include the matching version from a CDN, for example.

```
- if Rails.env.production?
  = javascript_include_tag "//ajax.googleapis.com/ajax/libs/jqueryui/#{Jquery::Ui::Rails::JQUERY_UI_VERSION}/jquery-ui.min.js"
- else
  = javascript_include_tag "jquery-ui"
```