add .gitlab-ci.yml

.gitlab-ci.yml

+---
+dune:git  clang:
+  image: duneci/dune:git
+  script: duneci-standard-test --opts=/duneci/opts.clang
+
+dune:git  gcc:
+  image: duneci/dune:git
+  script: duneci-standard-test
+
+dune:2.6  gcc:
+  image: duneci/dune:2.6
+  script: duneci-standard-test