Work around faulty EventMachine behavior

Defer detach to the next tick, because EventMachine blows up when a file
descriptor is attached and detached in the same tick. This can happen
when the child process dies in the same tick it started, and the
`#waitpid` loop in the signal handler picks it up afterwards. The signal
handler, in turn, queues the child's callback to the executed via
`EM#next_tick`. If the blocks queued by `EM#next_tick` are executed
after that, still in the same tick, the child's file descriptors can be
detached in the same tick they were attached.
Bump patch