Add .gitlab-ci.yml

.gitlab-ci.yml

+default:
+  image: maven:latest
+
+stages:
+  - build
+  - test
+  - diff
+
+build:
+  stage: build
+  image: maven:latest
+  script:
+    - mvn compile
+
+test:
+  stage: test
+  image: maven:3.6-jdk-8
+  script:
+    - mvn test
+
+deploy:
+  image: maven:3.6-jdk-8
+  script:
+    - 'mvn deploy -s ci_settings.xml'
+  only:
+    - master