Add actionable suggestion to activation error, refs #11 (thanks @renepickhardt!)

Author: shesek

woocommerce-gateway-lightning.php

@@ -15,8 +15,8 @@
 }
 
 register_activation_hook( __FILE__, function(){
-  if (!extension_loaded('gd') || !extension_loaded('curl')) {
-    die('The php-curl and php-gd extensions are required.');
+  if (!extension_loaded('gdx') || !extension_loaded('curl')) {
+    die('The php-curl and php-gd extensions are required. Please contact your hosting provider for additional help.');
   }
 });