Use https://get.docker.io/ubuntu consistently

The install script (on https://get.docker.io/) installs an APT sources.list
entry referencing an HTTPS repository, and takes care of installing the
apt-transport-https package. However, the Debian/Ubuntu specific installation
script (on https://get.docker.io/ubuntu) used an HTTPS repository but without
installing that package, causing the installation to fail on some platforms.

This will use HTTPS everywhere, and updates the documentation accordingly.

Docker-DCO-1.1-Signed-off-by: Jérôme Petazzoni <jerome@docker.com> (github: jpetazzo)