amend Rake 0.9 support

lib/bundler/gem_helper.rb

@@ -2,15 +2,10 @@
 require 'thor'
 require 'bundler'
 
-begin
-  # Support Rake > 0.8.7
-  require 'rake/dsl_definition'
-  include Rake::DSL
-rescue LoadError
-end
-
 module Bundler
   class GemHelper
+    include Rake::DSL if defined? Rake::DSL
+
     def self.install_tasks(opts = {})
       dir = opts[:dir] || Dir.pwd
       self.new(dir, opts[:name]).install