Skip clean all of ocaml

Fixes Homebrew/homebrew#188.

Signed-off-by: Max Howell <max@methylblue.com>

I tested just cleaning lib and the test cases still didn't work.

Formula/objective-caml.rb

@@ -17,5 +17,7 @@ class ObjectiveCaml <Formula
     system "make install"
   end
 
+  # note it indeed seems necessary to clean everything
+  # see http://github.com/mxcl/homebrew/issues#issue/188
   def skip_clean? path; true; end
 end