Use rdoc/task

- rake/rdoctask is obsolete.

Author: devn

Rakefile

@@ -13,8 +13,8 @@ end
 
 task :default => :spec
 
-require 'rake/rdoctask'
-Rake::RDocTask.new do |rdoc|
+require 'rdoc/task'
+RDoc::Task.new do |rdoc|
   version = File.exist?('VERSION') ? File.read('VERSION') : ""
 
   rdoc.rdoc_dir = 'rdoc'