Update GitLab CI runners

As discovered by @zj, https://gitlab.slack.com/archives/C3W3PSR88/p1532025703000084

We should update our GitLab CI runners in the Gitter internal network. They are currently running gitlab-ci-multi-runner 9.4.2 (released 2017-8-2)

• jenkins-slave-01
• jenkins-slave-02

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Docs: https://docs.gitlab.com/runner/install/

Todo

• Register runners on https://gitlab.com/
• Register runners on https://dev.gitlab.org/
  • The old jenkins-slave-01 and jenkins-slave-02 runners have been registered, https://dev.gitlab.org/gitlab/gitter/webapp/settings/ci_cd#js-runners-settings
• Add some old image cleanup maintenance cron to keep some disk space open ▶ https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/7711
  • Also add a monit check