update fs.close call in deviceserver.js

@falcone and I ran into an issue setting up an SRM-20 today. After running deviceserver.js, we can send a job, but at the end of the job an error is thrown:

The offending line is js/deviceserver.js:103 where we run fs.close.

According to https://nodejs.org/api/fs.html#fs_fs_close_fd_callback, the callback is no longer optional (as of V7.0.0, but now throws TypeError as of V10.0.0).

So, maybe something like:

fs.close(file,function(err) {
  if (err) {
    console.log('error: '+err)
    ws.send('error: '+err)
  }
});

?