Other valid modes if https:// does not work for you:
git clone http://github.com/raspberrypi/linux.git
git clone git://github.com/raspberrypi/linux.git
You may also be able to navigate with a browser to http://github.com/raspberrypi
and download an archive also, but I have not tried that.
Just tried all three modes (https, http and git) and as of 15:30 today, they were working for me.