passenger: Use opt_prefix

Cleaning up places that can use `opt_prefix` instead of
`#{HOMEBREW_PREFIX}/opt/<packagename>`

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/passenger.rb

@@ -30,8 +30,8 @@ def install
 
   def caveats; <<-EOS.undent
     To activate Phusion Passenger for Apache, create /etc/apache2/other/passenger.conf:
-      LoadModule passenger_module #{HOMEBREW_PREFIX}/opt/passenger/libout/apache2/mod_passenger.so
-      PassengerRoot #{HOMEBREW_PREFIX}/opt/passenger
+      LoadModule passenger_module #{opt_prefix}/libout/apache2/mod_passenger.so
+      PassengerRoot #{opt_prefix}
       PassengerDefaultRuby /usr/bin/ruby
 
     To activate Phusion Passenger for Nginx, run: