ruby: bump to version 2.1.2

This commit bumps Ruby to version 2.1.2. It was quickly runtime tested
on ARM EBIhf.

In addition to this it changes the site to an http:// location, which
is the official one advertised on the Ruby web site.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>