Do not use deprecated `util.pump`, it's going away.

[ChALkeR]

util.pump was deprecated since nodejs v0.10, and there is an ongoing discussion on removing that in the next major version: https://github.com/nodejs/node/pull/2531.

The lines in question: /lib/monitor.js#L76-81:

util.pump(readStream, res, function (e) {
    if(e) {
        console.error(e.stack || e);
    }
    res.end();
});

I am not making a pull request because I am not a user of this package and have not tested it.