Loosen cats-effect constraints

[kubukoz]

Touches #163 a bit. ConcurrentEffect replaced with Concurrent, ContextShift removed.

Let me know what you think about: replacing Timer usage with Clock (a Clock is implicitly available if a Timer is)

[purrgrammer]

First of all thanks for taking the time to improve Fetch, is greatly appreciated. I've pushed a branch called loosen-constraints with your commits so it's build by Travis with our config, it should be good to merge when tests are green 💚

Let me know what you think about: replacing Timer usage with Clock (a Clock is implicitly available if a Timer is)

👍

[kubukoz]

Superseded by #187