Explicitly require rake, fixing rake version compatibility

lib/annotate.rb

@@ -14,6 +14,7 @@ module Annotate
   
   def self.load_tasks
     if File.exists?('Rakefile')
+      require 'rake'
       load 'Rakefile'
       Dir[File.join(File.dirname(__FILE__), 'tasks', '**/*.rake')].each { |rake| load rake }
       return true