Fix for TypeError [ERR_INVALID_CALLBACK]: Callback must be a function…

[lankesh]

Fix for a bug filed in following github issue: Getting 'TypeError [ERR_INVALID_CALLBACK]: Callback must be a function. Received undefined' on tail.unwatch() #19

As per definition in https://nodejs.org/api/fs.html#fs_fs_close_fd_callback, fs.close expects a callback to be passed when calling it. But in the Tail.prototype.unwatch function, the callback is not passed and hence the function breaks when it gets called on the tail object.