Removed zend-diactoros as it is no longer dev dependency

Author: michalbundyra

composer.json

@@ -29,8 +29,7 @@
     "require-dev": {
         "phpunit/phpunit": "^6.5.5",
         "roave/security-advisories": "dev-master",
-        "zendframework/zend-coding-standard": "~1.0.0",
-        "zendframework/zend-diactoros": "^1.7"
+        "zendframework/zend-coding-standard": "~1.0.0"
     },
     "conflict": {
         "container-interop/container-interop": "<1.2.0"