Adapt to new structure of jQuery UI 1.11.0

Since version v5.0.0 (774f8b3) of `jquery-ui-rails` the file naming is updated:
jquery.ui.*.js/css => jquery-ui/*.js/css
Update jquery-ui-rails dependency in gemspec
Merge pull request #24 from koenpunt/patch-1

Adapt to new structure of jQuery UI 1.11.0