Update PHPUnit

Author: ondrejmirtes

composer.json

@@ -15,7 +15,7 @@
         "php-parallel-lint/php-parallel-lint": "^1.2",
         "phpstan/phpstan-phpunit": "^0.12",
         "phpstan/phpstan-strict-rules": "^0.12",
-        "phpunit/phpunit": "^7.2"
+        "phpunit/phpunit": "^7.5.20"
     },
     "config": {
         "platform": {