Updated composer/installers dependency

[mikelittle]

Up[dated composer/instlaller version to allow both 1.x and 2.x
Updated phpunit to allow it to run in php8+

[tomjn]

Looks like Travis is still running under PHP 7.2 hence the failing checks. I'd consider removing the PHPUnit change and specifying just the composer installs update. Otherwise we'll need to fix Travis

[tomjn]

Travis now runs the linter but the linter fails with PHPCS issues, it's likely the humanmade/coding-standards package need updating to fix this

[mikelittle]

Looks like Travis is still running under PHP 7.2 hence the failing checks. I'd consider removing the PHPUnit change and specifying just the composer installs update. Otherwise we'll need to fix Travis

I had to update PHPunit to get things to run under php8.3
I'll look at fixing the travis configuration.