Do not depend on http4s' cats-effect version transitively

[BenFradet]

It seems we're using the cats-effect version from http4s transitively: https://github.com/47degrees/github4s/pull/501#discussion_r433683082.

It'd be better to depend on it directly I think.

[cb372]

I hear there's an awesome sbt plugin called sbt-explicit-dependencies that can find issues like this for you