add manual pretender shutdown to fix acceptance tests

tests/helpers/destroy-app.js

@@ -1,5 +1,10 @@
 import run from 'ember-runloop';
 
 export default function destroyApp(application) {
+ // this is required to fix "second Pretender instance" warnings
+ if (server) {
+ server.shutdown();
+ }
+
  run(application, 'destroy');
 }