Upgrade gitlab-runner image tag to v13.1.0
This updates the gitlab-runner image tag to v13.1.0.
I did not see any breaking changes, that could affect the docker+machine setup, so it should run as before. Upgrading gitlab runner to v13.1.0 should be reasonable, as this version included many bug fixes and other improvements/features.
The changelog of gitlab runner can be found here: https://gitlab.com/gitlab-org/gitlab-runner/blob/master/CHANGELOG.md