Log Message:
Update ruby-rbtree to 0.4.0.

=== 0.4.0

* Fixed build failure with Ruby 2.0.0.
* \#bound now returns an enumerator if no block is given.
* \#select now returns a new MultiRBTree / RBTree.
* Fixed a bug where \#reject could return nil.
* \#to_s is now equivalent to \#inspect.
* \#each now passes a two elements array as an argument to the given block.
* Added new methods: \#default_proc=, \#flatten, \#keep_if, \#key, \#select!
  and \#to_h.