recaptcha: Update to Google recaptcha API v2 (Issue #313)

Recaptcha stopped working. It was using google recaptcha v1, and
https://developers.google.com/recaptcha/docs/faq says: "Any calls to the v1 API
will not work after March 31, 2018."

Note:

1) Not using 'async defer' - it's not used anywhere else, plus I have no idea
of what exactly it does.

2) The recaptcha div has an ID so the label for="recaptcha_field" can reference
it (although I'm not certain a div can technically have a label, but it's
better than removing the label, or the for="").