Update README.md for Node version requirement (#77)
Update README.md for Node version requirement (#77)

After scratching my head to figure out why `npm-register` was failing to launch, found that it is using several ES6 features which are only supported in Node 6.x or later. This update to the README makes that clear to other new users like myself :)
dickeyxxx/elephant,dickeyxxx/elephant,heroku/elephant,dickeyxxx/npm-register,heroku/elephant,dickeyxxx/npm-register,jdxcode/npm-register,jdxcode/npm-register