In my application.js file, I have:
//= require jquery
//= require jquery_ujs
//= require underscore
//= require backbone
//= require_tree .
//
//= require ./
You've still got a
//= require_tree .
Higher up, which is loading everything, apparently in alphabetical order. Remove that (obviously making sure that everything is required elsewhere) and you should be fine. You might be able to make that the last line of your application.js but I don't remember the specified behaviour when two statements end up requiring the same file