Update used Runner version to 11.3.0
This MR updates the used version of GitLab Runner as last step in Runner's release process. See gitlab-org/gitlab-runner#3579 (closed).
Additionally, since this MR bumps the version to >= 11.0, it also update one of used configuration keys that was changed in 11.0. This resolves #53 (closed).
The MR should be merged after Runners Pipeline for v11.3.0 becomes green: