Widen used PHPUnit version range include PHP 7.4 supporting version

Author: WyriHaximus

composer.json

@@ -20,7 +20,7 @@
   },
   "require-dev": {
     "clue/block-react": "^1.1",
-    "phpunit/phpunit": "^6.0 || ^5.0 || ^4.8"
+    "phpunit/phpunit": "^7.0 || ^6.0 || ^5.0 || ^4.8"
   },
   "autoload": {
     "psr-4": {