Use full path to du

Fixes Homebrew/homebrew#354

Library/Homebrew/extend/pathname.rb

@@ -93,7 +93,7 @@ class Pathname
     out=''
     n=`find #{to_s} -type f | wc -l`.to_i
     out<<"#{n} files, " if n > 1
-    out<<`du -hd0 #{to_s} | cut -d"\t" -f1`.strip
+    out<<`/usr/bin/du -hd0 #{to_s} | cut -d"\t" -f1`.strip
   end
 
   # attempts to retrieve the version component of this path, so generally