messagebus diagnostics process path on Mac OS

Currently, every time I run rails (on my Mac OS X), this warning is shown on the console:

    ps: %p|$|%a: keyword not found
    ps: no valid keywords; valid keywords:

This is caused by MessageBus diagnostics, which during init, tries to get current process' full path using

    info = `ps -eo "%p|$|%a" | grep '^\\s*#{Process.pid}'`

which uses Linux-specific syntax. To make it work on Mac OS too, I included a trivial OS check.