Fix Ubuntu version needed to use tails-installer

Hi,

sorry, but there is a mistake in the new installation assistant leading some Ubuntu users to send me bug reports and inquiries about tails-installer in Trusty:

https://tails.boum.org/install/debian/usb/index.en.html says:
“Ubuntu 14.04 (Trusty) or later in a PPA (Personal Package Archive)”

Unfortunately that is wrong. The installer is available only from Ubuntu Wily or Xenial, because Trusty lacks a recent version of syslinux.

Related issues

Related to #11017 (closed)
Related to #11004 (closed)

Original created by @u on 11012 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information