Suggestion: Change the default reCAPTCHA api host.

[Dolphinsimon]

As google.com is blocked in China, the reCAPTCHA service hosted by www.google.com doesn't work in China. But there is another host www.recaptcha.net for google's reCAPTCHA service can be accessed in China. For the service and all users the www.recaptcha.net host will be a more efficiency choice.