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.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3239 from sryza/sandy-spark-4375 and squashes the following commits:

0ffbe95 [Sandy Ryza] Enable -Dscala-2.11 in sbt
cd42d94 [Sandy Ryza] Update doc
f6644c3 [Sandy Ryza] SPARK-4375 take 2