Add missing zend-diactoros dependency

Author: geerteltink

composer.json

@@ -24,7 +24,8 @@
         "psr/container": "^1.0",
         "psr/http-message": "^1.0.1",
         "psr/http-server-handler": "^1.0",
-        "psr/http-server-middleware": "^1.0"
+        "psr/http-server-middleware": "^1.0",
+        "zendframework/zend-diactoros": "^1.7"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.5.5",