Changes the fs call to watchFile from watch so Cesium will load

@davideaster @eric79 
I don't know a lot about the nodejs server but a google search with the error message suggested the following change.  After updating the file, agi/cesium did load, and loaded quickly. 
This [blog](http://tech.nitoyon.com/en/blog/2013/10/02/node-watch-impl/) suggests that `watch` is newer and preferred but it is not supported on all platforms ... including Cygwin.  Do you run your server from Cygwin?