[Security] Bump phpunit/phpunit from 5.5.4 to 6.0.8

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 5.5.4 to 6.0.8. **This update includes security fixes.**
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/6.0.8/ChangeLog-6.0.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/6.0.8)

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

Author: dependabot-support

composer.json

@@ -11,7 +11,7 @@
     "php": "^7.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^5.2.3",
+    "phpunit/phpunit": "^6.0.8",
     "squizlabs/php_codesniffer": "^3.4",
     "phake/phake": "^2.3"
   },