Handle SIGUSR1 and SIGUSR2 as special cases so Sidekiq API works in JRuby. Fixes #2063

:coffee: 
I guess an equivalent statement would be

```
case msg
  when 'USR1', 'USR2'
      Sidekiq::CLI.instance.handle_signal(msg)
  else
     ::Process.kill(msg, $$) 
end
```

Actually, that's better, I'll change it. :smile: