write gz file to disk so docs build script can pick it up [ci skip]

build/grunt-tasks/build.js

@@ -28,7 +28,7 @@ module.exports = function (grunt) {
  zlib.gzip(buf, function (err, buf) {
  if (err) return done(err)
  report('dist/vue.min.js.gz', buf)
- done()
+ fs.writeFile('dist/vue.min.js.gz', buf, done)
  })
  })
  }