refine build config

config/index.js

     env: require('./prod.env'),
     index: path.resolve(__dirname, '../dist/index.html'),
     assetsRoot: path.resolve(__dirname, '../dist'),
-    assetsSubDirectory: './static',
+    assetsSubDirectory: 'static',
     assetsPublicPath: './',
     productionSourceMap: true,
     // Gzip off by default as many popular static hosts such as