nginx: fix passenger detection and module location

--root changed to an ini file but the check was looking for a directory

Closes Homebrew/homebrew#25743.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/nginx.rb

@@ -30,10 +30,10 @@ class Nginx < Formula
   skip_clean 'logs'
 
   def passenger_config_args
-    passenger_root = `passenger-config --root`.chomp
+    nginx_ext = `passenger-config --nginx-addon-dir`.chomp
 
-    if File.directory?(passenger_root)
-      return "--add-module=#{passenger_root}/ext/nginx"
+    if File.directory?(nginx_ext)
+      return "--add-module=#{nginx_ext}"
     end
 
     puts "Unable to install nginx with passenger support. The passenger"