Update composer.json

Author: adityapatadia

composer.json

@@ -36,7 +36,7 @@
     },
     "require-dev":
     {
-        "phpunit/phpunit": "^5.7",
+        "phpunit/phpunit": "^8.5",
         "apigen/apigen": "^4.1",
         "php-coveralls/php-coveralls": "^2.1",
         "ext-exif": "*",