You can't use a local URL. This is because a webhook request is a POST request 
that is made from our server. Therefore, the only URLs that we can make these 
requests to would be URLs that are publicly available.

For testing purposes, I would recommend using RequestBin. You can create a new 
bin and paste the URL for that bin in the webhook URL field of your Instamojo 
link. This will give you a link that is accessible by our server and you can 
inspect the POST requests to this link by appending ?inspect at the end of the 
URL.

Or just replace 

'webhook' => 'https://test.instamojo.com/api/1.1/'