Can't find line causing Deprecation warning

My Rails/Ember app is currently using ember version 1.5.1. I want to bring it to the last version of Ember and I'm doing so by updating one version at a time so I can get all deprecation warnings and fix them before moving on. When I try to go from version 1.6.1 to version 1.7.1 I get the following deprecation warning but I can't find anywhere in my app where I use then on an instance of Application.

Do you have any tips on how can I investigate and find where this use of then is happening? The provided stack trace is not being of any use for me.

DEPRECATION: Do not use `.then` on an instance of Ember.Application.  Please use the `.ready` hook instead.
    at Namespace.extend.then (http://localhost:3000/assets/ember.js?body=1:2861:15)
    at apply (http://`localhost:3000`/assets/ember.js?body=1:18384:27)
    at superWrapper [as then] (http://localhost:3000/assets/ember.js?body=1:17961:15)
    at tryThen (http://loDEPRECATION: Do not use `.then` on an instance of Ember.Application.  Please use the `.ready` hook instead.
    at Namespace.extend.then (http://localhost:3000/assets/ember.js?body=1:2861:15)
    at apply (http://localhost:3000/assets/ember.js?body=1:18384:27)
    at superWrapper [as then] (http://localhost:3000/assets/ember.js?body=1:17961:15)
    at tryThen (http://localhost:3000/assets/ember.js?body=1:45661:14)
    at http://localhost:3000/assets/ember.js?body=1:45670:21
    at http://localhost:3000/assets/ember.js?body=1:29072:9
    at DeferredActionQueues.invoke (http://localhost:3000/assets/ember.js?body=1:635:18)
    at Object.DeferredActionQueues.flush (http://localhost:3000/assets/ember.js?body=1:685:15)
    at Object.Backburner.end (http://localhost:3000/assets/ember.js?body=1:148:27)calhost:3000/assets/ember.js?body=1:45661:14)
    at http://localhost:3000/assets/ember.js?body=1:45670:21
    at http://localhost:3000/assets/ember.js?body=1:29072:9
    at DeferredActionQueues.invoke (http://localhost:3000/assets/ember.js?body=1:635:18)
    at Object.DeferredActionQueues.flush (http://localhost:3000/assets/ember.js?body=1:685:15)
    at Object.Backburner.end (http://localhost:3000/assets/ember.js?body=1:148:27)