cleans only existing directories

The previous code works fine on ruby 1.8.x, but under 1.9 trying
find on a non-existent folder gives:
  ==> No such file or directory
  /usr/local/Cellar/ruby/1.9.1-p378/lib/ruby/1.9.1/find.rb:38:in `block in find'

Fixes Homebrew/homebrew#1633

Library/Homebrew/brew.h.rb

@@ -478,9 +478,7 @@ end
 class Cleaner
   def initialize f
     @f=f
-
-    [f.bin, f.sbin, f.lib].each {|d| clean_dir d}
-
+    [f.bin, f.sbin, f.lib].select{|d|d.exist?}.each{|d|clean_dir d}
     # info pages suck
     info = f.share+'info'
     info.rmtree if info.directory? and not f.skip_clean? info