Bump Spark version to 1.6.1; use Scala 2.10.6
Bump Spark version to 1.6.1; use Scala 2.10.6

See also:

- http://www.scala-lang.org/news/2.10.6/
- https://spark.apache.org/releases/spark-release-1-6-1.html

azavea/docker-spark