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.
IMO, it is okay to add an exception. We sort of decided on same flags for both build to simplify things for sbt users(and may be maven users too). Its okay for things we don't have choice and should not forfeit the purpose. I will let Patrick comment too.