Travis: require PHPUnit 9.3 for PHP 8

PHPUnit 9.3 is not yet released (expected end of August), but is slated to add official PHP 8 support to PHPUnit.

As some tests are failing due to the new reserved keyword `match` being used in PHPUnit, I'm (temporarily) upping the PHPUnit requirement for PHP 8 explicitly to PHPUnit 9.3 (dev).

Once PHPUnit 9.3 has been released, this condition can be removed in favour of changing the requirements in the `composer.json` file.

Note: for now the `--ignore-platform-reqs` is still needed as the underlying dependencies may still not have tagged a version which officially supports PHP 8 and this would cause a requirement conflict.

Ref: sebastianbergmann/phpunit#4373

.travis.yml

@@ -211,6 +211,8 @@ install:
   # The Composer PHPCS plugin takes care of setting the installed_paths for PHPCS.
   - |
     if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
+      # Temporary fix - PHPUnit 9.3 is supposed to officially support PHP 8.
+      composer require --no-update phpunit/phpunit:"^9.3"
       # PHPUnit 9.x does not (yet) allow for installation on PHP 8, so ignore platform
       # requirements to get PHPUnit 9.x to install on nightly.
       composer install --prefer-dist --no-suggest --ignore-platform-reqs