Add GraphicsMagick to source install instructions
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14497 added a dependency on GraphicsMagick, and omnibus-gitlab!1975 (merged) added this to Omnibus. But @jacobvosmaer-gitlab pointed out that it's not in the source installation instructions.
@rymai pointed this out in gitlab-development-kit!480 (comment 80752380) but it looks like we missed adding that