Update Bunny to 0.10.4+

I believe RabbitMQ input/output do not need updating but
FTR, here's [the changelog](https://github.com/ruby-amqp/bunny/blob/0.10.x-stable/ChangeLog.md).
Merge pull request #599 from michaelklishin/patch-1

Update Bunny to 0.10.4+