Allow PHPUnit 5 to be installed for PHP 5.6

Author: dlh01

composer.json

@@ -15,7 +15,7 @@
   },
   "require-dev": {
     "alleyinteractive/alley-coding-standards": "^0.3.0",
-    "phpunit/phpunit": "^7.5",
+    "phpunit/phpunit": "^5.7.21 || ^7.5",
     "yoast/phpunit-polyfills": "^1.0"
   },
   "scripts": {