Fixed error with `make install`

rbinstall.rb couldn't load "psych/versions".

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56945 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/psych/psych.gemspec

@@ -1,6 +1,5 @@
 # -*- encoding: utf-8 -*-
-$LOAD_PATH.unshift './lib'
-load 'psych/versions.rb'
+require 'psych'
 
 Gem::Specification.new do |s|
   s.name = "psych"