.gitlab-ci.yml: update environments

Ansgar Burchardtrequested to merge
feature/update-ci-environments into master
  • 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

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