Widen PHPUnit versions to cover PHP 7.1 and above.

Author: pfrenssen

composer.json

@@ -10,7 +10,7 @@
         "scssphp/scssphp": "~1.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "~9",
+        "phpunit/phpunit": "~7 || ~8 || ~9",
         "slevomat/coding-standard": "~6",
         "squizlabs/php_codesniffer": "~3"
     },