Load plugin textdomain

Hi,

As per the notice at https://translate.ww.wp.xz.cn/projects/wp-plugins/really-simple-captcha, the plugin is not yet translatable. You’ve added the text domain plugin header (good), but you also need to load_plugin_textdomain(). With this done, the translation warning will disappear, and the plugin headers will become translatable.

Thanks,
Gary.