Fix warning

/home/travis/build/zzet/gitlabhq/app/models/commit.rb:137: warning: respond_to? is defined here
/home/travis/.rvm/gems/ruby-2.0.0-p353/gems/rspec-mocks-2.13.1/lib/rspec/mocks/extensions/marshal.rb:5: warning: Commit#respond_to?(:_dump) is old fashion which takes only one parameter