Require PHPUnit 7.5

Because assertStringContainsString was introduced in 7.5.0

Author: Naktibalda

composer.json

@@ -10,7 +10,7 @@
         }
     ],
     "require": {
-        "phpunit/phpunit": ">=7.1 <7.6",
+        "phpunit/phpunit": "7.5.*",
         "phpunit/php-code-coverage": "^6.0",
         "sebastian/comparator": "^3.0",
         "sebastian/diff": "^3.0"