Hook into rails-3 generators using 3.0 and 3.1 APIs

- Rails-3.1 will change the API for hooking into generators from
  "config.generators" to "config.app_generators".
- This commit supports either.

lib/rspec-rails.rb

@@ -1,8 +1,10 @@
 module RSpec
   module Rails
     class Railtie < ::Rails::Railtie
-      config.generators.integration_tool :rspec
-      config.generators.test_framework   :rspec
+      # Rails-3.0.1 requires config.app_generators instead of 3.0.0's config.generators
+      generators = config.respond_to?(:app_generators) ? config.app_generators : config.generators
+      generators.integration_tool :rspec
+      generators.test_framework   :rspec
 
       rake_tasks do
         load "rspec/rails/tasks/rspec.rake"