Use standard version of Promise.prototype.finally

Summary: See https://github.com/facebook/fbjs/pull/293 for an explanation.

Reviewed By: yungsters

Differential Revision: D14658680

fbshipit-source-id: aad8808b8514817314bf67b9a43b01d52e850e36

Libraries/Promise.js

@@ -11,11 +11,9 @@
 'use strict';
 
 const Promise = require('promise/setimmediate/es6-extensions');
-require('promise/setimmediate/done');
 
-Promise.prototype.finally = function(onSettled) {
-  return this.then(onSettled, onSettled);
-};
+require('promise/setimmediate/done');
+require('promise/setimmediate/finally');
 
 if (__DEV__) {
   require('promise/setimmediate/rejection-tracking').enable({