fix: implicitly nullable parameter type updated with explicit type

- adds github actions

- use published version (stdlib)

- version bump

- updates CI scripts

Merge pull request #17 from kodedphp/shmop

bugfix/2.2

dev/2.1.0 (#15)

* Fixing the unit and integration tests (for phpunit 7+)
* PHP min. requirement is 7.2
* Removed "--ignore-platform-reqs" for composer
* Version bump
* Badge update
* Removed support for PHP 7.1

- version bump

Updates (#12)

- FileClient creates now 1-level subdirectories (down from 2)
- improves the error handling in Redis/Json clients
- uses psr-4 for autoloading
- upgraded phpunit version
- updates some unit tests
- CS cleanup