fix($core): webpack externals (ref: #451)

Author: ulivz

packages/@vuepress/core/lib/node/webpack/createServerConfig.js

@@ -16,7 +16,7 @@ module.exports = function createServerConfig (ctx) {
 
   config
     .target('node')
-    .externals([/^vue|vue-router$/])
+    .externals([/^(vue|vue-router)$/])
     .devtool('source-map')
 
   // no need to minimize server build