Remove Debian 8 ("Jessie") from the GitLab CI
There are several reason why remove Debian 8 from the CI:
- Debian 8 ("jessie") has been superseded by Debian 9 ("stretch").
- Regular security support updates have been discontinued as of June 17th, 2018.
- Jessie LTS is supported from 17th June 2018 to June 30, 2020.
In other words, it's no longer officially supported by Debian security team, but by the volunteer/paid contributor composed LTS team. And the release will be discontinued in three months from now. We can use the freed CI resources to bring new platforms or just to make the jobs run a bit faster.
(cherry picked from commit 75f46cc3)