Upgrade to Finagle 6.24 and Scala 2.11

Dropped support for Scala 2.9, added cross-compilation for 2.10.4 and 2.11.4
This should probably go to: "Websudos releases" at "http://maven.websudos.co.uk/ext-release-local", have a look at where phantom/morpheus are published in our stack.