Fix Masterpiece Series: Kaladesh Inventions code
Hi,

you set `mp1` for masterpiece series but it seems to be mps (according to [mtgjson](
https://github.com/mtgjson/mtgjson/issues/273)).

<https://api.magicthegathering.io/v1/sets?name=inventions>

![image](https://cloud.githubusercontent.com/assets/4228646/23909216/72be5d86-08d6-11e7-81fc-a31d1e17885a.png)

It should be great if we could create alias for icons (such as mp1, mps, ...) in order to be backward compatible (if someone already hardcoded mp1 for Kaladesh Invention for instance).

I can help on the SASS edit (make a PR) if you want.
