Recent tickets have been opened regarding Scala 2.9 code that's preventing compilation on 2.11. (https://issues.apache.org/jira/browse/SAMZA-163, https://issues.apache.org/jira/browse/SAMZA-430). 2.9 came out in 2011 and at this point is pretty ancient.
Any opposition to removing support for 2.9 and focusing on 2.10 and above? I'd be +1. -jakob
