downgrade akka to 2.1.0

akka 2.2.x seems to have some major differences in how it handles tcp connections than what akka 2.0.5 (what is in master).  This caused a bunch of problems on deployments with slightly unreliable connections.  See the discussion on "executor failures w/ scala 2.10" here : <http://mail-archives.apache.org/mod_mbox/incubator-spark-user/201311.mbox/browser> (sorry no direct link)

downgrading to 2.1.0 seemed to restore behavior that we expected w/ akka 2.0.5 -- deploying to the same boxes worked reliably after this change.

(Notice that the diff is mostly making the akka code look the same as it does in master)