Merge pull request #198 from swistak/master

Preventing finalization on dead actor
Remove Celluloid.shutdown from Listen.stop
Stub Celluloid::SupervisionGroup#alive?
Debug travis
Removing signal handlers

Stopping Listen on SIGINT seems to be no longer equired.
Instead we provide a way to stop all listeners in a non-blocking
`Listen.stop` and blocking way `Listen.stop(:and_wait)`

It's recommended to call `Listen.stop(:and_wait)` before exiting your
application to finalize all listeners.