Bumping jersey-netty and kafka versions.

jersey-netty has been promoted from snapshot to proper version including
chunked transfers and some stability fix, kafka is now not a beta
anymore, which includes a working POM without the need to add all
transitive dependencies manually.
Merge branch '0.20' of github.com:Graylog2/graylog2-server into 0.20
Adding missing sonatype releases repo