Update .gitlab-ci.yml

1 job from master in 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #9481
build

00:00:11

 
Name Stage Failure
failed
build Build

[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 8.482 s
[INFO] Finished at: 2020-07-26T09:55:29+02:00
[INFO] ------------------------------------------------------------------------
bash: line 73: javac: command not found
ERROR: Job failed: exit status 1