Update from rake/rdoctask to rdoc/task

rake/rdoctask is deprecated

Rakefile

@@ -25,7 +25,7 @@ end
 
 task :default => :test
 
-require 'rake/rdoctask'
+require 'rdoc/task'
 Rake::RDocTask.new do |rdoc|
   if File.exist?('VERSION.yml')
     config = YAML.load(File.read('VERSION.yml'))