Google has changed the URLs.
You have to edit the file recaptcha.php
You can find it in plugins/captcha/recaptcha/recaptcha.php
There are three lines of code that need to be changed in recaptcha.php...... lines 22, 24 and 129.
See link below:
github.com/joomla/joomla-cms/pull/2437/files
Hope it helps