[SPARK-1812] Enable cross build for scala 2.11.

Was just trying out akka 2.3.3 (theoretically it should all work.)
Maintaining a separate branch is difficult and requires sometimes non trivial merges to keep it in sync. 
I don't really know, what an archetype build which generates pom(s) for different scala version mean.
I feel we can have some release scripts that can take care of releasing maven poms with expression in artifact replaced by its corresponding constant. This sort of thing sbt does natively. I think this is much better than complicating the build further, because once we have an archetype we will have to adjust sbt a great deal to accommodate it.