Update README.md to Refer users to active fork at alertifyjs/alertify.js

Refer users to active fork at [alertifyjs/alertify.js](https://github.com/alertifyjs/alertify.js)

Thanks for creating and sharing alertify.js! I've no affiliation with the fork mentioned above, but having tried it out, can vouch for its suitability as a drop-in continuation/replacement for the now-retired original. Modifications to the README are just the simplest rewrite I though appropriate provided for your convenience in case you approve of the nomination.

Yours gratefully,
Jonathan