Compatibility/security: Replace file_get_contents with cURL function

[SGOI]

Many web servers now force allow_url_fopen=0 and allow_url_include = 0 due to security concerns and this creates an incompatibility with the form (PHP warning: file_get_contents(): https:// wrapper is disabled in the server configuration by allow_url_fopen=0 in) when accessing the Google Captcha URL.