Update refs to callbacks for Fiber compat

[jaredpalmer]

Out of the box, when running a jest snapshot with React 16 alpha, I got this error:

TypeError: getStackAddendumByWorkInProgressFiber is not a function

      at captureError (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:889:29)
      at commitAllWork (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:400:9)
      at completeUnitOfWork (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:522:11)
      at performUnitOfWork (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:546:14)
      at workLoop (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:666:26)
      at performWork (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:707:30)
      at scheduleUpdate (../node_modules/react-test-renderer/lib/ReactFiberScheduler.js:1060:15)
      at scheduleTopLevelUpdate (../node_modules/react-test-renderer/lib/ReactFiberReconciler.js:60:5)
      at Object.updateContainer (../node_modules/react-test-renderer/lib/ReactFiberReconciler.js:90:7)
      at Object.create (../node_modules/react-test-renderer/lib/ReactTestRendererFiber.js:237:18)

This PR mitigates this error by replacing all string refs with callbacks.

[jwineman]

Thanks!

[jaredpalmer]

Should we bump dependency to include 16 too?

https://github.com/cloudflare/react-modal2/blob/master/package.json#L31