Update supported node version badge in README

[meeber]

Still says >=0.10 instead of >=4

[keithamus]

This should be a case of amending the README.md from:

    <img
      alt="node:?"
      src="https://img.shields.io/badge/node-%3E=0.10-blue.svg?style=flat-square"
    />

to

    <img
      alt="node:?"
      src="https://img.shields.io/badge/node-%3E=>4.0-blue.svg?style=flat-square"
    />

Anyone who would like to take this on, please announce it at file a pull request and we'll happily merge!