Remove Debian 8 ("Jessie") from the GitLab CI

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* Debian 8 ("jessie") has been superseded by Debian 9 ("stretch").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.