add manual pretender shutdown to fix acceptance tests (#477)

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

@@ -2,6 +2,11 @@ import run from 'ember-runloop';
 import $ from 'jquery';
 
 export default function destroyApp(application) {
+    // this is required to fix "second Pretender instance" warnings
+    if (server) {
+        server.shutdown();
+    }
+
     // this is required because it gets injected during acceptance tests but
     // not removed meaning that the integration tests grab this element rather
     // than their rendered content