shutdown pretender server on app destroy

This fixes a "You created a second Pretender instance
while there was already one running" warning during
acceptance tests.

See miragejs/ember-cli-mirage#915

live/tests/helpers/destroy-app.js

@@ -2,4 +2,5 @@ import Ember from 'ember';
 
 export default function destroyApp(application) {
   Ember.run(application, 'destroy');
+  server.shutdown();
 }