Do not publish aggregator projects, see #3481

With this change the only file that is generated for the parent projects is ivy.xml, e.g.
.ivy2/local/com.typesafe.akka/akka-samples/2.2-SNAPSHOT/ivys/ivy.xml
I assume that is all fine. I can't see it on maven central: http://mirrors.ibiblio.org/maven2/com/typesafe/akka/akka-samples/2.2.0-RC2/