Build instructions are broken on Jessie

The following packages have unmet dependencies:
vagrant-libvirt : Depends: ruby-fog-libvirt (>= 0.3.0) but it is not going to be installed

ruby-fog-libvirt/stretch depends on Stretch’s Ruby, that IMO we should not try installing on Jessie.

IMO the best fix is to simply require Stretch.

Original created by @intrigeri on 12655 (Redmine)

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