Require phpunit

Author: snapshotpl

composer.json

@@ -16,7 +16,7 @@
  "http-interop/http-middleware": "^0.4.1"
  },
  "require-dev": {
- "phpunit/phpunit": "^5.6 || ^6.1.3",
+ "phpunit/phpunit": "^5.7.27 || ^6.1.3",
  "ramsey/uuid": "^3.0",
  "zendframework/zend-diactoros": "^1.1.3"
  },