Remove override of object_id #77

Avoids this warning on 1.9.3:

    /Users/mat/.rvm/gems/ruby-1.9.3-p194@.../gems/hashie-2.0.0/lib/hashie/mash.rb:80: warning: redefining `object_id' may cause serious problems
Merge pull request #81 from matschaffer/issue/object_id-warning-77

Remove override of object_id #77