Added attr_accessible to Version

Attempt to correct issue with protected_attributes gem
when rails application is configured to the default

    # Enforce whitelist mode for mass assignment.
    config.active_record.whitelist_attributes = true

Error message is:

An error occurred in an after hook
  ActiveModel::MassAssignmentSecurity::Error:
    Can't mass-assign protected attributes for Version: event, whodunnit
  occurred at ~/.rvm/gems/ruby-1.9.3-p194/gems/protected_attributes-1.0.2/
    lib/active_model/mass_assignment_security/sanitizer.rb:60:
  in `process_removed_attributes'