Add timeout to stop killer after 5 seconds

@mbj this commit adds a timeout to long running mutations that have potentially created an infinite loop.

I realize this might not actually work in some cases, like when running --rspec-unit and --rspec-full but I wanted to start discussion on it somewhere since I just saw a case in ice_nine where a mutation became infinite.

I wonder if simply making the timeout configurable would be enough? Then if someone has a long running spec suite, they could make the timeout 2x longer than whatever it takes to run. In my case, since I'm using --rspec-dm2 I would set the timeout to 1 second or something low since no example takes anywhere near that much time to run.