Fix cannot sort problem caused by bootstrap shown event not fire.

app/views/admin/ad_banners/show.html.erb

@@ -117,7 +117,7 @@
   <% end %>
   <script type="text/javascript">
     var sortUpdated = false;
-    $("#sort-images-modal").on("shown",function(){
+    $(document).on('show.bs.modal','#sort-images-modal', function () {
       $(".order-list").sortable({
         update : function(){
           sortUpdated = true;