Add gitlab CI.

.gitlab-ci.yml

+---
+# Install external dependencies
+
+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