Enable React Constant Elements

[ForbesLindesay]

It looks like all the issues referenced by https://github.com/facebookincubator/create-react-app/blob/master/packages/babel-preset-react-app/index.js#L136 are closed now.

[gaearon]

We are tracking this in #553.

We still need to wait for Babel 7 for babel/babel#5415, but there may be more bugs. Will also need to test if babel/babel#5577 still reproduces.