fix loaded path order. This affects to use system rdoc

Author: hsbt

rdoc.gemspec

@@ -1,5 +1,5 @@
 # -*- encoding: utf-8 -*-
-$:.push File.expand_path("../lib", __FILE__)
+$:.unshift File.expand_path("../lib", __FILE__)
 require 'rdoc'
 
 Gem::Specification.new do |s|