Bump akka-actor version to 2.3.3 (ref SI-8549)

Because serialization affected akka, we're upgrading the included
akka-actor to 2.3.3, which is built with Scala 2.11.1.
Merge pull request #3762 from adriaanm/bump-akka

Bump akka-actor version to 2.3.3 (ref SI-8549)