Updated for the `conversations` API

[jlumbroso]

@andxyz Thanks for updating and formalizing the original gist! 🙏🏻😊

Since then, the Slack API has deprecated the calls to the channels, im, mpim, groups — and has replaced it all by a unified conversations API. I made the exact modifications that represent this change in API so that this script continues to work as intended.

I tested the script myself and it works as expected.