Allow using latest package with react v16

[nikolaik]

Also add a new test command to run tests against react v16 using npm run test:react16

[vercel[bot]]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/tbleckert/react-select-search/2XDQ5qcj716sUfT52unkmQpwRAJv
✅ Preview: https://react-select-search-git-fork-nikolaik-fix-react16-tbleckert.vercel.app

[tbleckert]

Perfect, thank you!