Update phpunit/phpunit requirement from ^4.8.6 to ^5.7.27

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/5.7.27/ChangeLog-5.7.md)
- [Commits](sebastianbergmann/phpunit@4.8.6...5.7.27)

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

Author: dependabot-preview[bot]

composer.json

@@ -8,7 +8,7 @@
         "psr/http-message": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.6"
+        "phpunit/phpunit": "^5.7.27"
     },
     "autoload": {
         "psr-4": {