Update sprockets. (#4167)

* Update sass-rails.

* FIX: Tilt dependency has been removed from Ember::Handlebars::Template.

* Update `DiscourseIIFE` to new Sprockets API.

* `Rails.application.assets` returns `nil` in production.

* Move sprockets-rails out of the assets group.

* Pin ember-rails to 0.18.5 which works with Sprockets 3.x.

* Update sprockets to 3.6.0.

* Make `DiscourseSassCompiler` work with Sprockets 3.

* Use `Sass::Rails::SassImporterGlobbing` instead of haxxing our own.

* Moneky patch so that we don't add dependencies for our custom css.

* FIX: Missing class.

* Upgrade ember-handlebars-template.

* FIX: require path needs to share the same root as the folder's path.

* Bump discourse-qunit-rails.

* Update ember-template-compiler.js to 1.12.2.

* `prepend` is private in Ruby 2.0.0.

test/javascripts/test_helper.js

@@ -1,7 +1,7 @@
 /*global document, sinon, QUnit, Logster */
 
 //= require env
-//= require ../../app/assets/javascripts/preload_store
+//= require preload_store
 //= require probes
 //= require jquery.debug
 //= require jquery.ui.widget
@@ -14,8 +14,8 @@
 //= require route-recognizer
 //= require pretender
 
-//= require ../../app/assets/javascripts/locales/i18n
-//= require ../../app/assets/javascripts/locales/en
+//= require locales/i18n
+//= require locales/en
 
 //= require vendor
 
@@ -39,7 +39,7 @@
 //= require plugin_tests
 //= require_self
 //
-//= require ../../public/javascripts/jquery.magnific-popup-min.js
+//= require jquery.magnific-popup-min.js
 
 window.inTestEnv = true;