active model tests can't be run

Currently:

(in /Users/apatterson/git/rails/activemodel)
/Users/apatterson/git/rails/activesupport/lib/active_support/test_case.rb:10: undefined method `silence_warnings' for main:Object (NoMethodError)
    from /Users/apatterson/git/rails/activemodel/lib/active_model/test_case.rb:2
    from /Users/apatterson/git/rails/activemodel/test/cases/attribute_methods_test.rb:24
    from /Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5:in `load'
    from /Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5
    from /Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5:in `each'
    from /Library/Ruby/Gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb:5
rake aborted!
Command failed with status (1): [/System/Library/Frameworks/Ruby.framework/...]

(See full trace by running task with --trace)

Attached patch fixes it.