Bump chokidar version to fix #81 and fix #82

Prior to chokidar 0.9, removing a watcher (for instance `w.close()` in `watchify#invalidate()`) will remove **all** watcher from anywhere (global `fs.unwatchFile()`). 

See [fix: Closing a chokidar instance prevent other instances to watch a file](https://github.com/paulmillr/chokidar/pull/150)

Maybe this fix is related to #81 and #82