Add Support for PHP 8

[IlliaBalia]

Please, consider updating composer.json file to

"require": {
        "php": "^5.5.9|^7.2|^7.3|^8.0",
        ...
}

in order to support PHP 8

[daniser]

Also, there's no sense in ^7.3 constraint here. 7.3 and 7.4 already covered by ^7.2 constraint.

[rodriguezaitor]

Please update this @SachinAgarwal1337

[SachinAgarwal1337]

Done in #71