Allow PHPUnit 7.0 to test on PHP 7.1

Author: schmengler

composer.json

@@ -16,7 +16,7 @@
   },
   "require-dev": {
     "pds/skeleton": "^1.0",
-    "phpunit/phpunit": "^8.0.5",
+    "phpunit/phpunit": "^7.0|^8.0",
     "phpro/grumphp": "^0.15.0",
     "squizlabs/php_codesniffer": "^3.4",
     "maglnet/composer-require-checker": "^2.0",