Update .gitlab-ci.yml to use cache

.gitlab-ci.yml

-image: maven:3-jdk-8
+ache:
+ paths:
+  - .m2/
+
+variables:
+ MAVEN_OPTS: "-Dmaven.repo.local=.m2"
+
+image: maven:3.5.3-jdk-8
 
 mvn:package:
   script: mvn clean package