Update composer.json

Author: GrahamCampbell

composer.json

@@ -10,7 +10,7 @@
         }
     ],
     "require": {
-        "php": "^7.1.3",
+        "php": "^7.2.5",
         "psr/http-message": "^1.0",
         "psr/cache": "^1.0",
         "php-http/httplug": "^1.1|^2.0",