message: Update Test badge in README
diff: <br />
  <br />
  - <a href="https://travis-ci.com/FormidableLabs/urql">
  - <img alt="Test Status" src="https://api.travis-ci.com/FormidableLabs/urql.svg?branch=master" />
  + <a href="https://circleci.com/gh/FormidableLabs/urql">
  + <img alt="Test Status" src="https://circleci.com/gh/FormidableLabs/urql.svg?style=shield" />
  </a>
  <a href="https://github.com/FormidableLabs/urql#maintenance-status">
  <img alt="Maintenance Status" src="https://img.shields.io/badge/maintenance-active-green.svg" />
  