Merge pull request #4412 from inez/master

Transition.then - it's optional, really is, to pass "onRejected" callback

ember/ember.d.ts

@@ -58,7 +58,7 @@ declare module EmberStates {
           @arg {String} label optional string for labeling the promise. Useful for tooling.
           @return {Promise}
          */
-        then(onFulfilled: Function, onRejected: Function, label?: string): Ember.RSVP.Promise;
+        then(onFulfilled: Function, onRejected?: Function, label?: string): Ember.RSVP.Promise;
 
         /**
           Forwards to the internal `promise` property which you can