Hardcoded certificate in snsValidCertUrl doesn't exist

[danjohnson95]

I'm unable to test the implementation locally because the certificate URL hardcoded in snsValidCertUrl is returning a 404:

laravel-sns-events/src/Concerns/GeneratesSnsMessages.php

Line 29 in a36a61a

public static $snsValidCertUrl = 'https://sns.us-west-2.amazonaws.com/bar.pem';

I've gone down a bit of a rabbit hole trying to figure out what this URL should be, and unfortunately the AWS docs seem outdated:

• https://aws.amazon.com/sns/faqs/
  These docs indicate the URL should be http://sns.us-east-1.amazonaws.com/SimpleNotificationService.pem for the US East 1 region, but this also 404s.
• http://sns-public-resources.s3.amazonaws.com/SNS_Message_Signing_Release_Note_Jan_25_2011.pdf
  These docs also indicate the URL should be http://sns.us-east-1.amazonaws.com/SimpleNotificationService.pem

I'm not entirely sure what the fix should be here, but happy to raise a PR if you have any ideas.