updating akka dependency to 2.3.2

I'm not fully sure if akka 2.2 and akka 2.3 are fully binary compatible nor if chill-akka would have any problems with it. But we are on our way to upgrading to akka 2.3 and as this is one of our major dependencies (*thanks*), I wanted to make sure that it was okay too. And it was. So here you go.