Switch from `/ci/api/v1` to `/api/v4/` for Runner 9.0

Requirement for gitlab-ce#27585

added Doing label

@ayufan For GitLab Runner 9.0 should we require GitLab 9.0, or should we make Runner 9.X (up to for example 9.4) be compatible also with older GitLab versions? What do you think?

@tmaczukin Runner 9.0 should support only GitLab 9.0. We will still for some time backport more important bug fixes to 1.11 that will be compatible with 8.x and 9.x.

cc @markpundsack

Yeah, it's awesome if we can maintain backwards compatibility for both components, but it's reasonable to force people to upgrade GitLab and then the runner (or at the same time, of course).

mentioned in merge request !517 (merged)

added GitLab API label

closed via merge request !517 (merged)