Update .gitlab-ci.yml

.gitlab-ci.yml

-image: slucky31/javafx
+image: maven:3.6.3-jdk-8
 
 
 build:
     script:
         - apt-get update
-        - apt-get install -y --no-install-recommends maven
+        - apt-get install openjfx=8u161-b12-1ubuntu2 libopenjfx-java=8u161-b12-1ubuntu2 libopenjfx-jni=8u161-b12-1ubuntu2
         - mvn package -B
         
 release: