FIX brew install before Cache has yet been created

Closes Homebrew/homebrew#7

Cache directory is created when downloading tarballs, but the new
hw.model stuff compiles the hw.model.c file there.

Library/Homebrew/hw.model.rb

@@ -1,4 +1,6 @@
 def hw_model_output
+  require 'fileutils'
+  FileUtils.mkpath HOMEBREW_CACHE
   exe=Pathname.new(HOMEBREW_CACHE)+'hw.model'
   Kernel.system "gcc -Os #{File.dirname __FILE__}/hw.model.c -o #{exe}" unless exe.file?
   /(.*)(\d+),(\d+)/ =~ `#{exe}`