Use gitlab-workhorse 0.6.1 with GitLab 8.4.0

We build extra executables in workhorse now. They should automatically end up in /opt/gitlab/embedded/bin.

@jacobvosmaer No longer necessary to do manual update of versions for gitlab-rails, gitlab-shell, gitlab-workhorse! All covered with the release-tools release-tools!4 (merged) and changes in omnibus-gitlab !605 (merged) :)

Only necessary to maintain correct version files in gitlab-rails!