Update documentation related to Workhorse

@rspeicher wrote in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2616#note_3412149:

Looks like we forgot to update docs for this change. The 8.3-to-8.4 guide still had 0.6.1 and the install guide still had 0.5.4

cc @jacobvosmaer @axil

I see this is fixed by @rspeicher :) https://gitlab.com/gitlab-org/gitlab-ce/commit/ca171b8190623023f3e6d00146abee8651cec857

And he is so awesome that picked it into 8-4-stable as well

Status changed to closed

Great! Thanks @rspeicher, @axil.

Yeah, sorry, I should have mentioned at the end of that note that I also updated it. :)