do not cleanup load path for jruby

jruby does not add default gems explicitly to the load path and the cleaning
of the load path wipes out the default laod_path of jruby in cases where
JRUBY_HOME == GEM_HOME

fixes rubygems#4565

lib/bundler/shared_helpers.rb

@@ -192,7 +192,8 @@ def set_rubylib
 
     def clean_load_path
       # handle 1.9 where system gems are always on the load path
-      if defined?(::Gem)
+      # jruby does not put any system gems explicitly on the load path
+      if defined?(::Gem) and not defined?(JRUBY_VERSION)
         me = File.expand_path("../../", __FILE__)
         me = /^#{Regexp.escape(me)}/