versions: unload old class before obtaining version

This is silly, but I am no Rubyist and I don't have time to
figure out why Formula.factory(foo).url works as expected but
Formula.factory(foo).version does not.

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

Library/Homebrew/cmd/versions.rb

@@ -62,6 +62,8 @@ class Formula
       mktemp do
         path = Pathname.new(Pathname.pwd+"#{name}.rb")
         path.write text_from_sha(sha)
+        # FIXME: shouldn't have to do this?
+        Object.send(:remove_const, "#{name.capitalize}")
         Formula.factory(path).version
       end rescue nil
     end