only check for our processes
only check for our processes

by @simcha
I got no errors on my local run and travis got different errors on my copy of same tree:
https://travis-ci.org/simcha/parallel/builds/168296730.
It seams to be error related to process_diff method
And especially command "ps uaxw|grep ruby|wc -l"
It is not checking only our processes and sometime can result with error.

grosser/parallel