SPARK-4375. no longer require -Pscala-2.10

It seems like the winds might have moved away from this approach, but wanted to post the PR anyway because I got it working and to show what it would look like.
I'm not very familiar with the sbt build, but yeah, that instruction only applies to maven.  Do we try to make all the same options have the same effect?  I could add a line under the sbt that explains how to use sbt with Scala 2.11.