Try to use different maven image, one with JavaFX, for issue #7

.gitlab-ci.yml

-image: maven:3.3.9-jdk-8
+image: kaiwinter/docker-java8-maven
+# image: maven:3.3.9-jdk-8
 
 # most of this taken from https://stackoverflow.com/questions/37785154/how-to-enable-maven-artifact-caching-for-gitlab-ci-runner