Library / language upgrades. Fixed issue with RaftClusterActor

+ Upgraded to Akka 2.3.8
+ Added cross compiling to Scala 2.11.4
+ Fixed an issue with the RaftClusterActor that caused the Leader to spam itself with AppendEntries messages after an election. I created a simple example to demonstrate how I was using RaftClusterActor and you can see the issue in action on the "broken" branch of [bradegler/akka-raft-example](https://github.com/bradegler/akka-raft-example/tree/broken)
Yes, this fixes the particular bug I was interested in. There is only one other reference to membersExceptSelf in the Candidate trait when starting an election. Filtering out the self member seemed to have no effect on the election logic so I stuck to the single change in the place that was having the issue to avoid any unintended side effects in Candidate.