Make library compatible with babashka.

[borkdude]

This PR makes cli-matic compatible with babashka. Fixes #113.

Changes:

• Use java.time instead of SimpleDateFormat to parse a date.
• Add bb.edn for running tests with babashka + cognitect test runner (also in CI).
• To check availability of core.async, test using (require 'clojure.core.async.impl.channels): this specific namespace isn't exposed in bb so you cannot use it in this library for bb, but it should still work for JVM Clojure. It seems this impl namespace is only necessary for checking if something is a channel. I wonder if core.async could support that use case better.
• Tests that didn't work with bb (yet) were marked with ^:skip-bb.
• Upgrade YAML library to fix failing lein tests (unrelated to babashka).

[borkdude]

/cc @nikvdp