chore: prevent banner from being removed by minifiers (#1851)

Trying to debug a webpack bundle splitting setup was made a bit trickier because it was hard to tell where this package was being included. Adding this should make the process a bit simpler.

Author: scottbedard

build/configs.js

@@ -6,7 +6,7 @@ const node = require('rollup-plugin-node-resolve')
 const replace = require('rollup-plugin-replace')
 const version = process.env.VERSION || require('../package.json').version
 const banner =
-`/**
+`/*!
   * vue-router v${version}
   * (c) ${new Date().getFullYear()} Evan You
   * @license MIT