.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

.gitlab-ci.yml

@@ -2,6 +2,10 @@
 before_script:
   - duneci-install-module https://gitlab.dune-project.org/core/dune-common.git
 
+debian:10  gcc:c++17:
+  image: duneci/base:10
+  script: duneci-standard-test --opts=/duneci/opts.gcc.c++17
+
 debian:9--gcc:
   image: duneci/base:9
   script: duneci-standard-test
@@ -14,10 +18,6 @@ debian:8--gcc:
   image: duneci/base:8
   script: duneci-standard-test
 
-debian:8-backports--clang:
-  image: duneci/base:8-backports
-  script: duneci-standard-test --opts=/duneci/opts.clang
-
 ubuntu:16.04--gcc:
   image: duneci/base:16.04
   script: duneci-standard-test