Fix middleware stops hanging on JRuby
Fix middleware stops hanging on JRuby

Raising Peribot::Middleware::Stop would cause a hang on JRuby where no
problems occurred on MRI. This probably had something to do with my use
of break instead of next to exit the promise's rescue block, though
admittedly I don't know enough about concurrent-ruby to know exactly
why the behavior was different. In any case, it is fixed now.

ahamlinman/peribot