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

Was just trying out akka 2.3.3 (theoretically it should all work.)
OK, I agree. I just inspected the generated pom, and the artifactId is
literally the string "spark_assembly_${scala.binary.version}". Till now I
always thought that along with the warning the variable is evaluated too,
but this clearly does not work at all.