Refs object is not extensible

Writing a ref causes:
```javascript
Uncaught TypeError: Cannot add property <a>, object is not extensible
```

I believe that refs is a frozen object now.  Related issue facebook/react#7136.

To fix this, I propose that we just write the refs directly to `this` as suggested in the linked issue above. I am open to other ideas though.

react@16.0.0
purescript-react@5.0.0