Update SDK library link in readme.txt

The original library http://github.com/facebook/php-sdk/

has been moved to a new location https://github.com/facebook/facebook-php-sdk

This version of the PHP SDK has been deprecated.

We have created a new repository for this project: https://github.com/facebook/facebook-php-sdk.

To see the repository in its previously published state, please see the deprecated branch.