When updating Passenger:

sudo gem update passenger
passenger-install-apache2-module

The second command is important as it compiles the apache 2 module (caught me out for a whiles, hence this post). Ensure that the module is properly added to your config file. (e.g. /etc/apache2/apache2.conf) by confirming its pointing to the new directory for the updated version of the gem! An example is given in the blurb one the apache2 module is compiled:

LoadModule passenger_module /usr/lib/ruby/gems/1.8/gems/passenger-2.2.15/ext/apache2/mod_passenger.so
PassengerRoot /usr/lib/ruby/gems/1.8/gems/passenger-2.2.15
PassengerRuby /usr/bin/ruby1.8