ci: introduce multi build runners (#36)

* docs: remove scrutinizer

* docs: add badges

* ci: upgrade ci

* build: upgrade dependencies

* test: upgrade test runners

* build: php 8.1 compatibility

Docs update & version bump (#24)

* docs: update example code

* build: update dependencies

* ci: update workflow

* ci: update scrutinizer

Upgrade dependencies (#4)

* Promise v1

* Badges

* Documentation

* Upgrade to phpgt/promise v2

* Backwards compatibility with PHP 7.4