Add implementation of 'includes' for PhantomJS

https://stackoverflow.com/a/31361163/429533

Author: jacebrowning

testing/test.js

@@ -2,6 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this file,
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+ if (!String.prototype.includes) {
+   String.prototype.includes = function() {'use strict';
+     return String.prototype.indexOf.apply(this, arguments) !== -1;
+   };
+ }
+
  QUnit.module( 'module', {
    beforeEach: function( assert ) {
       init();