Force php 7.1 or higher

Author: Shivella

composer.json

@@ -12,7 +12,8 @@
         }
     ],
     "require": {
-        "php": ">=5.4",
+        "php": ">=7.1.0",
+        "ext-json": "*",
         "symfony/framework-bundle": "^2.7 || ^3.0 || ^4.0 || ^5.0",
         "api-postcode/php-client": "^1.0"
     },