Update test env gitlab-shell version when v3.1.0 is published
In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4578/diffs#e2d0a8523084b34c7501ebd8dacb37aa5176cf64_84_83 we use a hard-coded version of gitlab-shell.
Remove it when v.3.1.0 (from https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/61) gets published.