Update chokidar to 0.9.0 to avoid install failure

Older version of chokidar (0.8.4) depends an outdated version of fsevent, which would [fails to install](https://github.com/pipobscure/fsevents/pull/26) on Node beta (v0.11+).

The latest version of chokidar rectifies this problem. And `npm test` runs fine with the new chokidar.