Add gitlab-workhorse to rake gitlab:check

What does this MR do?

It adds a check for the gitlab-workhorse version in the rake task "rake gitlab:check"

Are there points in the code the reviewer needs to double check?

I am assuming the needed version for gitlab-workhorse is 0.8.4.

Why was this MR needed?

dding a check for gitlab-shell version would be helpful to debian packaging (as all other dependencies are checked using Gemfile this get missed).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?