The error keeps generating every time an opportunity is won.  The ITFFF package is deactivated.  What is the issue and how  can it be resolved?

Apex script unhandled exception by user/organization: 0054P0000092zac/00DU0000000LVr8

Failed to invoke future method 'public static void callout(String, String)' on class 'ifttt.Webhook' for job id '7074P0000DOoUqx'

caused by: System.CalloutException: Unauthorized endpoint, please check Setup->Security->Remote site settings. endpoint = http://ifttt-salesforce.herokuapp.com/ifttt/v1/webhook/opportunity_was_won

Class.ifttt.Webhook.callout: line 32, column 1