fixed wrong number of arguments (2 for 1) for set

ArgumentError: wrong number of arguments (2 for 1)
/home/csolg/.rvm/gems/ruby-2.1.1/gems/mongoid-4.0.0.beta1/lib/mongoid/persistable/settable.rb:22:in `set'
/home/csolg/.rvm/gems/ruby-2.1.1/bundler/gems/mongoid_nested_set-bb67b010f30c/lib/mongoid_nested_set/update.rb:191:in `block in update_self_and_descendants_depth'
/home/csolg/.rvm/gems/ruby-2.1.1/bundler/gems/mongoid_nested_set-bb67b010f30c/lib/mongoid_nested_set/document.rb:57:in `block in each_with_level'