Updated reCAPTCHA code to the latest version

- Removed separate SSL link and using the protocol agnostic URL instead: `//
- Removed the unused `server` variable
- Updated the HTML code to the latest version from https://developers.google
- Updated tests
Removed SSL URL reference from docs
Merge pull request #145 from lipis/patch-2

Updated reCAPTCHA code to the latest version
Add RECAPTCHA_API_SERVER configuration. Fix RECAPTCHA verify server url.

https://github.com/lepture/flask-wtf/pull/145
Fix test cases
Add documentation on RECAPTCHA_API_SERVER