Keep the mana costs gone, but let Riven stack mana still so she can use Manamune

Riven is mostly in a very good spot atm, but she just is too weak in today's meta. I think part of this is that she can't build Manamune. Now, obviously she can't have mana costs as that would break her, but she should be able to still stack some so she can build mana-centered items still.

Rito pls