Add .gitlab-ci.yml

.gitlab-ci.yml

+.default job:
+    image: ubuntu:rolling
+    tags:
+        - docker
+    before_script:
+        - apt-get update
+        - apt-get -y install
+            g++ clang++
+    script:
+        - ${CXX} -std=c++14 visit_bools.test.cpp && ./a.out
+
+gcc:
+    extends: .default job
+    variables:
+        CXX: g++
+
+clang:
+    extends: .default job
+    variables:
+        CXX: clang++