remove echo to stdout in Factory.php

Hi!

I've a problem with `\BotMan\Drivers\Slack\Factory::createUsingRTM`.
I makes an `echo` to stdout. Line [71](https://github.com/botman/driver-slack/blob/master/src/Factory.php#L71) and [74](https://github.com/botman/driver-slack/blob/master/src/Factory.php#L74). 
Can you remove this? Or use some logging module for the output? Or a `debug` / `verbose` flag?

Thx!
Georg.
