Fix use of Ember global

Fixes #827

Author: simonihmig

packages/ember-cli-fastboot/vendor/experimental-render-mode-rehydrate.js

@@ -1,6 +1,7 @@
 (function() {
   if (typeof FastBoot === 'undefined') {
     var current = document.getElementById('fastboot-body-start');
+    var Ember = require('ember').default;
 
     if (
       current &&