fix js es6

+'use strict';
 
-let cdnDomains = require('../../../../config/dns-prefetch');
+var cdnDomains = require('../../../../config/dns-prefetch');
 
 function cdnReplace(cdn, imgUrl) {
     if (cdn === 'qcloud') {
@@ -13,13 +14,13 @@ function cdnReplace(cdn, imgUrl) {
 }
 
 module.exports = function (imageUrl, opts) {
-    let cdn = window.cdn;
+    var cdn = window.cdn;
 
     if (imageUrl) {
-        let params = opts.hash;
-        let urls = imageUrl.split('?');
-        let query = urls[1] || '';
-        let uri = urls[0];
+        var params = opts.hash;
+        var urls = imageUrl.split('?');
+        var query = urls[1] || '';
+        var uri = urls[0];
 
         if (uri.indexOf('http:') === 0) {
             uri = uri.replace('http:', '');