Laravel 8 support?

I managed to install botman on Laravel 8. Telegram and Microsoft Bot drivers are also successfully installed. But there are problems with the Slack driver.

```
Installing driver "Slack"
Using version ^2.2 for botman/driver-slack

./composer.json has been updated

Loading composer repositories with package information

Updating dependencies (including require-dev)

Your requirements could not be resolved to an installable set of packages.

Problem 1
- Installation request for botman/driver-slack ^2.2 -> satisfiable by botman/driver-slack[2.2.0].
- Can only install one of: guzzlehttp/guzzle[7.1.1, 6.5.x-dev].
- Can only install one of: guzzlehttp/guzzle[6.5.x-dev, 7.1.1].
- Can only install one of: guzzlehttp/guzzle[6.5.x-dev, 7.1.1].
- Conclusion: install guzzlehttp/guzzle 6.5.x-dev
- Installation request for guzzlehttp/guzzle (locked at 7.1.1, required as ^7.0.1) -> satisfiable by guzzlehttp/guzzle[7.1.1].

Installation failed, reverting ./composer.json to its original content.

Error installing driver "Slack"
```

Maybe someone was able to solve this problem?