amending package constraint

raising minimum phpunit version due to deprecated ReflectionType::__toString() usage.

Author: SignpostMarv

composer.json

@@ -21,7 +21,7 @@
     "phpunit/php-timer": "^2.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7.0",
+    "phpunit/phpunit": "^7.5.20",
     "squizlabs/php_codesniffer": "^2.8.1"
   },
   "autoload": {