Updated 'passenger-memory-monitor' for Passenger 4

Updated the 'passenger-memory-monitor' script to work with Passenger
4.0. This scripts now makes use of the passenger gem instead of
'passenger-status' to get pid and memory info for passenger processes.
This update has been tested against both Passenger 3 and 4.
Merge pull request #1 from bryantraywick/passenger-memory-monitor-4-support

Updated 'passenger-memory-monitor' for Passenger 4