Merge pull request #8 from Girgias/ext-json-dependency

Add JSON extension as a dependency.

Author: neilime

composer.json

@@ -27,7 +27,8 @@
         }
     },
     "require": {
-        "php": "^7.2"
+        "php": "^7.2",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^7 || ^8.2"