Update GITLAB_SHELL_VERSION to 5.1.1
Why
Update to >= 5.1.0 needed for gitlab-ee!2240 on EE.
Updated here to keep CE and EE on the same version.
5.1.1 includes gitlab-shell!142 (merged) to revert changes which were causing problems with Post Receive hooks seen in https://gitlab.com/gitlab-org/gitlab-ee/issues/2822.
Edited by James Edwards-Jones