update 还原webpack-server,处理下载依赖报错

Author: hzlshen

package.json

@@ -56,7 +56,7 @@
     "vue-template-compiler": "^2.5.21",
     "webpack": "^3.6.0",
     "webpack-bundle-analyzer": "^2.9.0",
-    "webpack-dev-server": ">=3.1.11",
+    "webpack-dev-server": "^2.9.1",
     "webpack-merge": "^4.2.1"
   },
   "engines": {