Adjust the laminas/laminas-dom constraints so older versions of PHP may still install 2.7.x

Author: stevegrunwell

composer.json

@@ -15,7 +15,7 @@
         "source": "https://github.com/stevegrunwell/phpunit-markup-assertions/"
     },
     "require": {
-        "laminas/laminas-dom": "^2.7"
+        "laminas/laminas-dom": "^2.7.0|^2.8"
     },
     "require-dev": {
         "symfony/phpunit-bridge": "^5.2"