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

Was just trying out akka 2.3.3 (theoretically it should all work.)
After some trial, I tend to agree with @ScrapCodes, that if it is just a warning we could probably ignore it and use variable names in artifactids. Keeps things a lot simpler that way. Besides, it really doesn't seem like an ugly hack to use variables in artifactid, somehow maven seems to be unhappy for unknown reason.