Update GITLAB_SHELL_VERSION to 5.1.1

Update to >= 5.1.0 needed for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2240 on EE.
Updated here to keep CE and EE on the same version.
5.1.1 includes https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/142 to revert changes which were causing problems with Post Receive hooks.

GITLAB_SHELL_VERSION

-5.0.6
+5.1.1