Update README.md

Fix links to Facebook Platform PHP5 SDK and ReCAPTCHA PHP library.

**Facebook Platform PHP5 SDK**
Old: https://github.com/facebook/php-sdk -> 404
New: https://github.com/facebook/facebook-php-sdk -> Works!
> Note: https://github.com/facebook/facebook-php-sdk-v4 is the latest version.

**ReCAPTCHA PHP library**
Old: http://recaptcha.net/plugins/php/ -> 404
New: https://developers.google.com/recaptcha/docs/php -> Works!
> Note: http://recaptcha.net/ redirects to https://developers.google.com/recaptcha/