readme: Update badges

* Use img.shields.io. This has existed for a while now and all the
  popular services either host this software, proxy it, or link it
  directly. Let's use this for badges going forward to keep them uniform.

* Link directly to npmjs.org instead of via badge.fury.io redirect.

* Fix tooltip to (un)capitalise npm correctly.

* Add Packagist badge.

Change-Id: I40154e7e5995160d7995c16882c7163f994ac0c6