Is public key necessary?

[darrenklein]

Thank you so much for your hard work on this very useful library. Is there a reason that we need to provide the reCAPTCHA public key? I may be mistaken, but it doesn't appear that it's actually used in any of this library's core logic, nor do I think it's a necessary component for validating a reCAPTCHA value. Perhaps you had a reason for including it that I don't understand.

If I'm correct, I'd be more than happy to open a PR to address that.