Fix for Rubinius 2.0

I ran into a problem where this gem wasn't working when I wanted to switch to Rubinius, so I tracked this down and fixed it.

Since this relies on monkeypatching a function (`_parse`), that a user wouldn't normally call, and internally Rubinius uses a different arity on `_parse`, I think this patch is probably a good idea.  : )  What do you think?