BREAKING CHANGE: UPGRADE REACT NATIVE TO BABEL 7!

Summary:
BREAKING CHANGE
This change upgrades the React Native build pipeline from Babel 6 to Babel 7

If you use a `.babelrc` then you'll need to update it to Babel 7 (note that some plugins are no longer relevant, some plugins are automatically upgraded, and some will need some manual love).

Note that you may also need to upgrade your dev env, tests etc, to make sure they work with Babel 7.

Reviewed By: cpojer

Differential Revision: D7097279

fbshipit-source-id: 9fb204cae733174a1c155669b7c17ddb70f7aecc