Zombie process check

I have been using bluepill to manage multiple processes on our Heroku dyno. In doing this I ran into issues where by a process crashed, but was not cleared up properly, leaving a zombie process.

I have added a new process check in order to cater for this scenario. The check will only work correctly if the process has cache_actual_pid set to false. I added a comment in the ZombieProcess class stating this but wasn't sure about adding to the README.