fix Es6 code issue.

This file is not transpiled, so we should stick to ES5

Author: zhenhappy

template/build/dev-client.js

@@ -2,7 +2,7 @@
 
 /* eslint-disable */
 require('eventsource-polyfill')
-const hotClient = require('webpack-hot-middleware/client?noInfo=true&reload=true')
+var hotClient = require('webpack-hot-middleware/client?noInfo=true&reload=true')
 
 hotClient.subscribe(function (event) {
   if (event.action === 'reload') {