F# MapReduce uses InternalActorRef. It should be using ActorRef. Breaking change

`InternalActorRef` is only meant to be used internally by Akka, and not exposed externally, so I changed it to `ActorRef`.
Upcoming PR for #182 will change `ActorOf` to return an `ActorRef` (which will change the signature of `spawns`). This is to prepare for that commit.

@Horusiath Can you verify this PR is OK (tests working, but my F# skills could be better :)