GitLab CI setup file in scratch/tzz/gitlab

[Ted Zlatanov]

I've pushed a .gitlab-ci.yml file to the branch scratch/tzz/gitlab

Merging that branch and letting me and Toon maintain that file would
make it much easier to support https://gitlab.com/emacs-ci/emacs and
further evaluate its viability as a CI service.

John, Eli, any objections? I can add a disclaimer, if you wish, that the
presence of the file doesn't imply support or endorsement of GitLab. I
don't think it's necessary personally. Right now it just names me as the
maintainer.

Thanks
Ted