Update tree.rb

permit the use of a Proc to render each line of print_tree
example of use:
````
tree.root_node.print_tree 0, method(:print_node)
def print_node(node, prefix)
  # your code
end
````
allow pre 1.9 rubies
Updated the dependencies (gems).
Merged the print_tree improvements.

This will allow an optional block to be added to the print_tree method.

See pull request https://github.com/evolve75/RubyTree/pull/26.

Merge branch 'net1957-master'
Merge branch 'next'
Updated the release date for the gem file.