v2.9.4+ Requires React 16

[LucasUnplugged]

Describe the bug
Starting with v2.9.4, react-datepicker requires React 16. This is because commit #1898 makes a change to calendar.jsx, which includes a call to React.createRef, which is only a method in v16 of React.

If you could update the Compatibility section of the readme.md file, that'd be great. Alternatively, if you'd like to maintain compatibility with React 15, you may be able to do so by finding an alternative to using React.createRef.

[martijnrusschen]

Fixed!

[macpham]

FYI @martijnrusschen I think that change should have incremented the major version. With the change mentioned above, the API to display the calendar broke in React 15.x which is public facing API of your repo that would have worked for apps using React 15.x with react-datepicker 2.9.3.

According to semantic versioning, that would be classified as a major change since it is not backwards compatible.

E.g. Apps that include react-datepicker with a ~2.9.3 or ^2.9.3, picking up patch and patch/minor versions respectively, would have included this change which semver would classify as major since backwards compatibility of opening the calendar was broken for React 15.x which it was previously working.

[martijnrusschen]

I agree with this. This has been an oversight on our end, as we didn't check backward compatibility before releasing this change.

[macpham]

No problem, thanks for this repo!