Add PHPunit ^7.0

Author: Jordan Hall

composer.json

@@ -25,7 +25,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.0",
+        "phpunit/phpunit": "^7.0||^8.0",
         "php-coveralls/php-coveralls": "^2.0"
     }
 }