fixes chunking for subclasses of GELF::Notifier

To subclass GELF::Notifier and be able to use chunking, you have to explicitly initialize last_chunk_id.  It looks to me like this variable belongs to GELF::Notifier, and using the explicit class name instead of self.class makes it work correctly when subclassed.

If I've misunderstood the scope of last_chunk_id, I'll modify my site's usage to correctly initialize.