.gitlab-ci.yml: update environments
- Added: Debian 10 with gcc in C++17 mode
- Removed: Debian 8 + Backports with clang
Reference: http://lists.dune-project.org/pipermail/dune-devel/2017-July/002224.html
Edited by Ansgar Burchardt
Reference: http://lists.dune-project.org/pipermail/dune-devel/2017-July/002224.html