Add Packagist latest version badge

Similar to the version badges at https://poser.pugx.org/, example: ![symfony release](https://poser.pugx.org/symfony/symfony/v/stable.png)

Finds the latest unstable or stable version from `package.versions`. Colours similarly to NPM (`dev-master/0.*` is orange, `>=1.*` is blue).

Supposedly versions are in descending order on the Packagist response but I'm not sure I trust JSON to stay in order. Possible optimisation but Symfony has a bunch of versions and is fairly quick to render.

Given I don't work with Node that often, please let me know of any sins I have committed!