Re-order update steps in the 8.14 -> 8.15 upgrade guide

1. Gems need to be installed before we can run the
  gitlab:workhorse:install task
1. Update gitlab-shell after gitlab-workhorse since it's a manual step
  that needs to be done in `/home/git/gitlab-shell`

Signed-off-by: Rémy Coutable <remy@rymai.me>