InternalPool doesn't clean up at_exit

Any threads from the InternalPool that aren't currently allocated to actors will block forever, causing JRuby runtimes to be retained in hot deployment environments.  The problematic line appears to be (from 0.13.0):

https://github.com/celluloid/celluloid/blob/bcff8ec7f8d77a9a51c314c35578654095c64a70/lib/celluloid/internal_pool.rb#L53