refs #7227 - set PassengerRuby to ruby1.9.1 on Ubuntu 12.04

Explicitly set PassengerRuby from the passenger_ruby parameter and set it to
ruby1.9.1 on Ubuntu 12.04 to ensure Passenger runs under the intended version
if the system-wide default is changed.

Deprecates the passenger_scl parameter, which has been split into the
passenger_ruby and passenger_ruby_package parameters for more control.

Clarify the support policy which has been followed anyway, and begin to
document how to use N-1 support.