He estado buscando por todas partes para resolver este problema. Ya bajé la gema jquery-rails a 2.3.0 en mi gemfile. Pero cuando incluyo jquery-ui en application.js, todavía obtengo el error Sprokets :: FileNotFound.

couldn't find file 'jquery-ui'
  (in /home/richard/projects/pickaxe/mini-projects/depot-app/app/assets/javascripts/application.js:14)

Este es mi archivo application.js:

// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below.
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, vendor/assets/javascripts,
// or vendor/assets/javascripts of plugins, if any, can be referenced here using a relative path.
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// the compiled file.
//
// WARNING: THE FIRST BLANK LINE MARKS THE END OF WHAT'S TO BE PROCESSED, ANY BLANK LINE SHOULD
// GO AFTER THE REQUIRES BELOW.
//
//= require jquery
//= require jquery-ui
//= require jquery_ujs
//= require_tree .

Me muero por saber por qué esto no funciona ...