Replace SIGKILL signal handling by SIGTERM and SIGINT

[ssideleau]

Trying to listen for the SIGKILL IPC signal results in a uv_signal_start EINVAL error.

According to nodejs documentation on signal events:

'SIGKILL' cannot have a listener installed, it will unconditionally terminate Node.js on all platforms.

Listening on SIGINT (ctrl + c) and SIGTERM should be enough to terminate the sort process gracefully.

Thank you for the library it works great once that bug is taken out of the equation.

[nelson-perez]

Thanks for the change.