Switched main page and download site to SSL

Letsencrypt is in public beta now. As promised I switched the main page as well as the download site to SSL. The main page redirects to HTTPS and therefore cannot be accessed without SSL anymore.

The Letsencrypt certificate causes the main page to not work with Windows XP anymore. That are 2,6% of all visitors to that page. They are working on a fix for that issue, though. Let me know if there are any other problems.

By always being on a SSL-protected site you can be certain you are getting the official UrBackup binaries even if an attacker is able to modify your network traffic.