Add support for composer/installers v2

[goldenapples]

This plugin is pinned to composer/installers 1.7, and this is the only blocker from using composer/installers v2 on an Altis project.

https://github.com/humanmade/PHPBasicAuth/blob/master/composer.json#L28

Can this requirement be updated to include ^2?

[tomjn]

@goldenapples see #29 it looks like you're not alone