small build-doc update
update maven dependency for ERJGroupsSynchronizer

As stated by Peter Vandoros on Wonder-disc mailinglist, ERJGroupsSynchronizer
needs JGroups 2.6 to work. So set the version in pom.xml to the library version
used in ERJGroupsSynchronizer framework.
Fix the dependency declaration for the new version of jgroups

Update the artifactId declaration with the correct name for this version. The version available in the central repository has a .GA suffix which is added by this commit too. Finally, a small fix for transitive dependencies of jgroups is removed as soon as it's not required anymore.