Composer dependencies issues

I spoke to @mpociot after his talk at LaraconEU and told him that I had some issues with the composer dependencies. After manually messing with them, I did get the driver to work perfectly. But not a very clean out-of-the-box experience, but perfect if you like dependency puzzles :-)

What I ended up with is this working combination:
```
"botman/botman": "^2.4",
"botman/driver-slack": "^2.1.3",
"evenement/evenement": "~3.0",
"mpociot/phpws": "^2.0",
"react/event-loop": "^0.4.3",
"react/stream": "^0.7.5",
```

Maybe you could look into updated everything, so manually downgrading would not be needed anymore.