Travis: Enable tests on PHP 8.1

[RoSk0]

[RoSk0]

I'm porting to use this project and notice test failure on PHP 8.1 in https://github.com/ingenerator/behat-tableassert/actions/runs/3067074223/jobs/4953045784 :

PHPUnit 9.5.0 by Sebastian Bergmann and contributors.

.............................II................................  63 / 123 ( 51%)
PHP Fatal error:  During inheritance of Iterator: Uncaught Return type of Masterminds\HTML5\Parser\StringInputStream::current() should either be compatible with Iterator::current(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

/home/runner/work/behat-tableassert/behat-tableassert/vendor/masterminds/html5/src/HTML5/Parser/StringInputStream.php:180
/home/runner/work/behat-tableassert/behat-tableassert/vendor/masterminds/html5/src/HTML5/Parser/StringInputStream.php:42
/home/runner/work/behat-tableassert/behat-tableassert/vendor/masterminds/html5/src/HTML5.php:92

[RoSk0]

Sorry, that test run actually used version 2.0.0 of this project so not a relevant complaint.

Also it doesn't look like Travis is in use anymore, but I will leave it open just in case I miss something.

[goetas]

replced by https://github.com/Masterminds/html5-php/pull/223