Include Diactoros 2.0 or later as a possible dependency

Author: lornajane

composer.json

@@ -17,7 +17,7 @@
   "require": {
     "php": ">=5.6",
     "php-http/client-implementation": "^1.0",
-    "zendframework/zend-diactoros": "^1.8.4",
+    "zendframework/zend-diactoros": "^1.8.4 || ^2.0",
     "php-http/guzzle6-adapter": "^1.0",
     "lcobucci/jwt": "^3.2"
   },