Since Dogma has been deprecated for over a year I’d recommend to only use credo and not dogma. Maybe dogma is having issues on Elixir 1.8+, but that seems unlikely since you’re getting an error from credo.
What version of Credo are you using? Are you using the latest version?
The Version.InvalidRequirementError sounds like this issue: Mix error corresponding to erlang-20rc.1 version but you shouldn’t be getting that error with Elixir 1.8.1 and OTP 21
