Add React 16 as peer dependency

[brycehill]

I didn't see an open issue/PR regarding React 16. All the tests passed and I didn't see any errors when using react-day-picker with React 16 in my app.

[codecov]

Codecov Report

Merging #498 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #498   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         534    534           
  Branches      111    111           
=====================================
  Hits          534    534

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[gpbl]

thanks!