display updated_at for app

app/app-detail.component.html

@@ -133,6 +133,9 @@
 
             <dt i18n>发行日期</dt>
             <dd>{{currentApp.released_at | date:'mediumDate'}}</dd>
+
+            <dt i18n *ngIf="currentApp.updated_at">更新日期</dt>
+            <dd *ngIf="currentApp.updated_at">{{currentApp.updated_at | date:'mediumDate'}}</dd>
         </dl>
     </div>
 </div>