add manual pretender shutdown to fix acceptance tests

no issue
- a [recent update](pretenderjs/pretender#178) to Pretender contained a breaking change that throws an error when multiple pretender instances exist
- using mirage in acceptance tests was [triggering the error](miragejs/ember-cli-mirage#915)

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');
 }