use shorter file hash

Author: yyx990803

template/build/webpack.base.conf.js

@@ -38,7 +38,7 @@ module.exports = {
         loader: 'url',
         query: {
           limit: 10000,
-          name: '[name].[ext]?[hash]'
+          name: '[name].[ext]?[hash:7]'
         }
       }
     ]