Fix dialyzer warnings on OTP21

[elbrujohalcon]

They look like this:

src/elvis.erl
 207: The pattern {'fail', _} can never match the type 'ok'

src/elvis_webhook.erl
  90: The pattern {'ok', Content} can never match the type {'error',_}
  98: The pattern {'ok', ConfigContent} can never match the type {'error',_}

And they only appear on OTP21. It has to be something related to both egithub and elvis_core, but I'm not sure what can it be.

[jfacorro]

Isn't this fixed?

[elbrujohalcon]

Yes, it is.