Made compatible with Babashka

[pekeler]

Babashka is great for writing scripts in Clojure. We use it for our CI and deployment scripts. I find it quite common wanting to send automated messages to Slack from our scripts.

clj-slack has two dependencies which are not compatible with Babashka: clj-http and data.json. They can be easily replaced with clj-http-lite and cheshire, both of which are compatible. This is what this PR is for.

lein test on your trunk isn't fully working for me. Not sure why. This PR doesn't add any additional failures or errors.

I'm using chat and reactions from my scripts now.