Running file_exists on a URL prefixed with "//" takes forever to run

When a filepath is prefixed with "//", Windows attempts to locate a **network share** with that address (which I'm guessing is the bottleneck).

E.g. //www.google.com/recaptcha/api/js/recaptcha_ajax.js

This patch skips the `file_exists` check if the $script starts with "//". There's probably a more elegant fix, such as using is_ssl() to force either "http:" or "https:"