Update the vue.js version to the latest

[defcc]

Currently https://vuejs.org/js/vue.js uses @2.4.0, and also we might need to make it automatically when deployed.

[Jinjiang]

node update.js needed. I have made a simple PR about that.
And it seems also need you clone vue repo beside this repo and keep vue repo at latest master branch: https://github.com/vuejs/vuejs.org/blob/87f1d8e395539750f2861c497796e7e011aef454/update.js#L3