Fix Linux OS name detection

Avoids problems like https://groups.google.com/forum/#!msg/phusion-passenger/IDacgpHxPiQ/iS2essCu_-UJ
Support Ruby 2.2 rb_thread_call_without_gvl()

Closes GH-1314.
Bump version to 4.0.55, update changelog
Merge branch 'stable-4.0'

Conflicts:
	CHANGELOG
	ext/common/Constants.h
	lib/phusion_passenger.rb