Fixed invite resend

refs 3d7e593c96

- `record.belongsTo().id` is an older Ember Data concept so it was returning `undefined`
- the `role` relationship is set up as `async: false` and is always output as an embedded record by our API so we're safe to use `this.role.id` without dealing with any async lookup

ghost/admin/app/models/invite.js

@@ -21,7 +21,7 @@ export default Model.extend({
     resend() {
         let inviteData = {
             email: this.email,
-            role_id: this.belongsTo('role').id
+            role_id: this.role.id
         };
 
         let inviteUrl = this.get('ghostPaths.url').api('invites');