Add an attr_reader for 'head'

This allows `brew audit` to inspect the head URLs, for example; until
now, that check was failing silently.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/formula.rb

@@ -100,7 +100,7 @@ class Formula
   include FileUtils
 
   attr_reader :name, :path, :url, :version, :homepage, :specs, :downloader
-  attr_reader :bottle, :bottle_sha1
+  attr_reader :bottle, :bottle_sha1, :head
 
   # Homebrew determines the name
   def initialize name='__UNKNOWN__', path=nil