docs: function find(ports, callback) {
    if (_.isEmpty(ports)) {
      callback(new Error('no free port available'));
    } else {
      var port = ports.shift();
      checkPort(port, function (err, found) {
        if (!err) {
          callback(null, found);
        } else {
          find(ports, callback);
        }
      });
    }
  }