AWS SNS not sending subscription confirmation for HTTP

I am trying to set up a basic SNS subscription but it looks like AWS never sends the confirmation, either through the console or using the SDK.

Steps I have taken so far:

• I have set up a Topic using the AWS console
• I have added HTTP subscriptions to both an ngrok tunnel URL and also a public web server URL using both the AWS console and the ruby SDK
• I have allowed everyone to subscribe to the Topic by editing the Topic policy in the AWS console

By tailing the logs on both my localhost and public web servers I can see that no request is ever sent to the endpoint.

I'm pretty stumped here because it seems like it should definitely be working, or at least sending something (anything) to my endpoints?

The code I'm using for the ruby sdk is as follows:

sns = AWS::SNS.new(access_key_id: ENV['AWS_ACCESS_KEY_ID'], secret_access_key: ENV['AWS_SECRET_ACCESS_KEY'])

# I only have one topic
topic = sns.topics.first

topic.subscribe(URI.parse('http://ift.tt/1TNIfzU'))

topic.subscriptions.first

# returns: => <AWS::SNS::Subscription arn:PendingConfirmation>