[Improvement] upgrade to webpack 4 (#693)

build/webpack.config.prod.js

@@ -4,19 +4,12 @@ const path = require('path');
 const devConfig = require('./webpack.config.dev.js');
 
 module.exports = merge(devConfig, {
+  mode: 'production',
   output: {
     path: path.join(__dirname, '../docs/dist'),
     publicPath: 'https://img.yzcdn.cn/zanui/vant/',
     filename: '[name].[hash:8].js',
     umdNamedDefine: true,
     chunkFilename: 'async_[name].[chunkhash:8].js'
-  },
-  plugins: [
-    new webpack.DefinePlugin({
-      'process.env': {
-        NODE_ENV: JSON.stringify(process.env.NODE_ENV)
-      }
-    }),
-    new webpack.optimize.UglifyJsPlugin()
-  ]
+  }
 });