https://tails.boum.org/contribute/build/ fails by default

When following the instructions at https://tails.boum.org/contribute/build/ on a virgin Debian 8, it fails with:

debian@test-loic:~/tails$ sudo apt-get install \
    git \
    rake \
    libvirt-daemon-system \
    dnsmasq-base \
    ebtables \
    qemu-system-x86 \
    qemu-utils \
    vagrant \
    vagrant-libvirt \
    vmdebootstrap
...
The following packages have unmet dependencies:
 vagrant-libvirt : Depends: ruby-fog-libvirt (>= 0.3.0) but it is not going to be installed

Related issues

• Is duplicate of #12655 (closed)

Original created by @dachary on 12716 (Redmine)