brew-dirty: simplify implementation

Library/Contributions/cmd/brew-dirty.rb

 # See: http://github.com/mxcl/homebrew/issues/issue/1359
 
-to_list = HOMEBREW_CELLAR.children.select { |pn| pn.directory? }
-to_list.each do |d|
-  versions = d.children.select { |pn| pn.directory? }.collect { |pn| pn.basename.to_s }
-  puts "#{d.basename} (#{versions.join(', ')})" if versions.size > 1
+require 'keg'
+
+HOMEBREW_CELLAR.subdirs.each do |rack|
+  versions = rack.subdirs.map { |d| Keg.new(d).version }
+  puts "#{rack.basename} (#{versions.join(', ')})" if versions.size > 1
 end