修复某些情况下打包白屏的问题

在某些情况下webpack打包完成后会阻塞在控制台，导致gulp后续任务无法执行，因此打包不完整导致白屏。

build/build.js

@@ -10,6 +10,7 @@ const chalk = require('chalk')
 const webpack = require('webpack')
 const config = require('../config')
 const webpackConfig = require('./webpack.prod.conf')
+const { exit } = require('process')
 
 const spinner = ora('building for production...')
 spinner.start()
@@ -37,5 +38,6 @@ rm(path.join(config.build.assetsRoot, config.build.assetsSubDirectory), err => {
       '  Tip: built files are meant to be served over an HTTP server.\n' +
       '  Opening index.html over file:// won\'t work.\n'
     ))
+    exit(0) // 在某些情况下webpack会阻塞在控制台，导致gulp后续任务无法执行，需要手动退出
   })
 })