Make Mailbox#next_message private, closes #374
Make Mailbox#next_message private, closes #374

Calling #next_message without holding the mutex is unsafe. This method should not be part of Mailbox's public API.
seuros/celluloid,jasonm23/celluloid,dilumn/celluloid,jstoja/celluloid,celluloid/celluloid,davydovanton/celluloid,marshall-lee/celluloid,sideci-sample/sideci-sample-celluloid,kenichi/celluloid,olleolleolle/celluloid,TiagoCardoso1983/celluloid