Fixed gh-profile-image tests

no issue
- stubbed `ghostPaths` service did not have the expected `ghostPaths.assetRoot` property that the `gh-profile-image` component started calling `.replace` on in adb0508d2f

ghost/admin/tests/integration/components/gh-profile-image-test.js

@@ -10,6 +10,8 @@ import {setupComponentTest} from 'ember-mocha';
 import {timeout} from 'ember-concurrency';
 
 let pathsStub = Service.extend({
+    assetRoot: '/ghost/assets/',
+
     init() {
         this._super(...arguments);