Update phpunit/phpunit requirement from ^4.8.36 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@4.8.36...5.7.27)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -19,7 +19,7 @@
         "ext-iconv": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8.36",
+        "phpunit/phpunit": "^5.7.27",
         "codacy/coverage": "^1.4.3"
     },
     "suggest": {