use pss when calculating master and worker memory

This PR uses the new PSS mem_type [submitted as a PR](https://github.com/schneems/get_process_mem/pull/2) for get_process_mem.

puma_worker_killer will automatically use PSS if on a Linux system.

There may be times when PSS probably isn't a good solution. I'm thinking of a setup where the machine is not dedicated to puma, as this might skew the PSS results, but I don't see how RSS would be any better in that case either.