Move the finalizer to the private interface.

Because of the yard documentation, it was confusing on how to terminate
a group when you should just use #terminate instead which is included
from Celluloid. This should not be called via the public interface as
it's meant to be a callback when this actor is terminated.

Change-Id: I222ed445f0cfd7aa9763ffbc3ca288c8b8a1f67b
Merge pull request #397 from Senjai/privatize_finalizer

Move the finalizer to the private interface.