Update broken link in README

[severin]

Seems like the auth0.com blog has messed with its URLs: https://auth0.com/blog/2015/03/31/critical-vulnerabilities-in-json-web-token-libraries/ results in a 404 but I assume this is the article that should be linked: https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/

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[anakinj]

Thanks for the fix!