Update Supported OS doc

As discussed in the mail list (Subject: Dropping support for Quantal), we need to update:

https://leap.se/en/docs/client#Supported.OSs

Support through debian packages:

• Ubuntu 13.10
• Ubuntu 14.04
• Debian 7?
• any other?

Support through bundle: the ones that are already listed in the doc.

(from redmine: created on 2013-12-23, closed on 2014-03-14, relates #5300 (closed))