Revert adding the zend-diactoros dependency

As explained by others, there is a test in place before using
Response(). If the class does not exist an exception is thrown.

Author: geerteltink

composer.json

@@ -9,7 +9,8 @@
         "http",
         "middleware",
         "zend-expressive",
-        "psr-7"
+        "psr-7",
+        "psr-15"
     ],
     "support": {
         "docs": "https://docs.zendframework.com/zend-expressive-authentication/",
@@ -23,8 +24,7 @@
         "php": "^7.1",
         "psr/container": "^1.0",
         "psr/http-message": "^1.0.1",
-        "psr/http-server-middleware": "^1.0",
-        "zendframework/zend-diactoros": "^1.7"
+        "psr/http-server-middleware": "^1.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.5.5",