[REEF-2058] Update FindBugs to version 3.0.4 (#1490)
[REEF-2058] Update FindBugs to version 3.0.4 (#1490)

FindBugs plugin v 3.0.2 that we use does not work with newer versions of maven (3.6+). We need to upgrade FindBugs to version 3.0.4.

JIRA: [REEF-2058](https://issues.apache.org/jira/projects/REEF/issues/REEF-2058)
motus/reef,apache/incubator-reef,dongjoon-hyun/incubator-reef,apache/incubator-reef,dongjoon-hyun/reef,motus/reef,apache/incubator-reef,dongjoon-hyun/incubator-reef,motus/reef,motus/reef,apache/reef,motus/reef,apache/reef,dongjoon-hyun/reef,dongjoon-hyun/incubator-reef,apache/incubator-reef,dongjoon-hyun/incubator-reef,apache/incubator-reef,apache/reef,apache/incubator-reef,dongjoon-hyun/incubator-reef,dongjoon-hyun/incubator-reef,motus/reef,apache/reef,apache/reef,dongjoon-hyun/reef,motus/reef,apache/reef,dongjoon-hyun/reef,dongjoon-hyun/incubator-reef,dongjoon-hyun/reef,apache/reef,dongjoon-hyun/reef,apache/incubator-reef,dongjoon-hyun/reef