Update phpunit/phpunit requirement from ^5.0 to ^7.5

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/7.5.20/ChangeLog-7.5.md)
- [Commits](sebastianbergmann/phpunit@5.0.0...7.5.20)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -23,7 +23,7 @@
     },
     "require-dev": {
         "mockery/mockery": "~1.3",
-        "phpunit/phpunit": "^5.0",
+        "phpunit/phpunit": "^7.5",
         "squizlabs/php_codesniffer": "^3.6",
         "friendsofphp/php-cs-fixer": "~2.19",
         "jakub-onderka/php-parallel-lint": "~0.9",