Fix ensure_resource on directory tree for boxen brews

Version 1.9.1 pushed a change which creates `Library/Taps/boxen/hombrew-brews` except it assumes `Library/Taps/boxen` already exists.

Puppet doesn't implicitly create directory trees, you need to do it explicitly as per http://www.puppetcookbook.com/posts/creating-a-directory-tree.html