don't leak Server version for assets.gratipay.com

How it was found

Hi i found that server version are being disclosed on the response header on this URL: https://assets.gratipay.com , this is a low risk but you can consider this as best practice because it is important to keep secret of server versions.

See similar reports here: #141125

Feel free to close this as Informative if don't see any security risk by the given information disclosure.

Regards Japz