Adds ext-json as an explicit requirement

Author: weierophinney

composer.json

@@ -7,6 +7,7 @@
         "sort-packages": true
     },
     "require": {
+        "ext-json": "*",
         "fig/http-message-util": "^1.1.2",
         "http-interop/http-middleware": "^0.4.1",
         "php": "^7.1",