ignore rake load error

rake is not available until installation now, so skip rake
packagetask even if it cannot load until it is actually needed.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50191 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/test_case.rb

@@ -1439,7 +1439,10 @@ begin
 rescue Gem::LoadError
 end
 
-require 'rake/packagetask'
+begin
+  require 'rake/packagetask'
+rescue LoadError
+end
 
 begin
   gem 'rdoc'