fix($core): unexpected webpack externals (close: #451) (#1539)

Author: egoist

lib/webpack/createServerConfig.js

@@ -11,7 +11,7 @@ module.exports = function createServerConfig (options, cliOptions) {
 
   config
     .target('node')
-    .externals([/^vue|vue-router$/])
+    .externals([/^(vue|vue-router)$/])
     .devtool('source-map')
 
   // no need to minimize server build