Use gitlab-workhorse 0.6.0

GITLAB_WORKHORSE_VERSION

-0.5.4
+0.6.0

doc/update/8.3-to-8.4.md

@@ -48,7 +48,7 @@ which should already be on your system from GitLab 8.1.
 ```bash
 cd /home/git/gitlab-workhorse
 sudo -u git -H git fetch --all
-sudo -u git -H git checkout 0.5.4
+sudo -u git -H git checkout 0.6.0
 sudo -u git -H make
 ```