Checks PHP version bebore requiring GMP (#346)

Fixes #316

src/WebPush.php

@@ -69,10 +69,12 @@ public function __construct(array $auth = [], array $defaultOptions = [], ?int $
     {
         $extensions = [
             'curl' => '[WebPush] curl extension is not loaded but is required. You can fix this in your php.ini.',
-            'gmp' => '[WebPush] gmp extension is not loaded but is required for sending push notifications with payload or for VAPID authentication. You can fix this in your php.ini.',
             'mbstring' => '[WebPush] mbstring extension is not loaded but is required for sending push notifications with payload or for VAPID authentication. You can fix this in your php.ini.',
             'openssl' => '[WebPush] openssl extension is not loaded but is required for sending push notifications with payload or for VAPID authentication. You can fix this in your php.ini.',
         ];
+        if (version_compare(phpversion(), '7.3.0', '<')) {
+            $extensions['gmp'] = '[WebPush] gmp extension is not loaded but is required for sending push notifications with payload or for VAPID authentication. You can fix this in your php.ini.';
+        }
         foreach ($extensions as $extension => $message) {
             if (!extension_loaded($extension)) {
                 trigger_error($message, E_USER_WARNING);