Fix behaviour of DraftPublished and ChangeMerged

Draft published event should not implement RepositoryModifiedEvent since it
does not really modify the repo.

Change Merged has the potential the modify the repo so we must implement
RepositoryModifiedEvent.

[JENKINS-25047]
Merge pull request #25 from scoheb/replication-fixes

Fix behaviour of DraftPublished and ChangeMerged
Updated changelog
[maven-release-plugin] prepare release gerrit-events-2.4.0
[maven-release-plugin] prepare for next development iteration