bump phpunit5

Author: Sean O'Brien

composer.json

@@ -19,7 +19,7 @@
  "symfony/http-kernel": "~2.3|~3.0|~4.0|~5.0|~6.0"
  },
  "require-dev": {
- "phpunit/phpunit": "^4.8.35|^5.4.0|^6.0.0",
+ "phpunit/phpunit": "^4.8.35|^5.6.3|^6.0.0",
  "symfony/framework-bundle": "~2.3|~3.0|~4.0|~5.0|~6.0",
  "symfony/yaml": "~2.3|~3.0|~4.0|~5.0|~6.0"
  },