Use RHASH_SIZE instead of RHASH() to grab internals of Hash.

On Rubinius, Hash in implemented in Ruby. RHASH() grabs internals
of how Hash is layed out in MRI. This is behavior that can't be
emulated, but with macro's / functions like RHASH_SIZE this can be
done in a compatible way.
Merge pull request #7 from dbussink/patch-1

Use RHASH_SIZE instead of RHASH() to grab internals of Hash.