use the rails standard way of initializing a gem, instead of that RAILS_ROOT check

init.rb

@@ -1 +1 @@
-require 'shoulda/rails'
+require File.join(File.dirname(__FILE__), 'rails', 'init')

lib/shoulda.rb

@@ -3,7 +3,6 @@ require 'shoulda/proc_extensions'
 require 'shoulda/assertions'
 require 'shoulda/macros'
 require 'shoulda/helpers'
-require 'shoulda/rails' if defined? RAILS_ROOT
 
 module Test # :nodoc: all
   module Unit

rails/init.rb

@@ -0,0 +1 @@
+require 'shoulda/rails'