Use jquery-ui-rails for jQuery UI

Spree uses jquery-rails ~>3.0.0 as of commit spree/spree@ea5610378c11baafa85eea87543d563d982ceaab.
This version doesn't include jquery-ui, so jquery-ui-rails used instead.

Fixes #48

This was already done for `2-0-stable` in 831cdf41efb91f8e827e411571910c071bae407d