removes the documentation group from the Gemfile, thus making RDoc 2.2 required in dev machines

In order to generate the API with that particular version we need to activate the gem
in the Rakefile before rake. Therefore the Rakefile does not honor --without for that
group now. Installing rdoc is no big deal anyway.

Gemfile

@@ -5,6 +5,7 @@ gem "rails", :path => File.dirname(__FILE__)
 
 gem "rake",  ">= 0.8.7"
 gem "mocha", ">= 0.9.8"
+gem "rdoc",  "2.2"
 
 mri = !defined?(RUBY_ENGINE) || RUBY_ENGINE == "ruby"
 if mri && RUBY_VERSION < '1.9'
@@ -44,10 +45,6 @@ elsif RUBY_ENGINE == "jruby"
   end
 end
 
-group :documentation do
-  gem 'rdoc', '2.2'
-end
-
 if ENV['CI']
   gem "nokogiri", ">= 1.4.0"