Install busybox On Debian/stretch as it's required for initramfs

[mika]

Followup fix for 2708f44

Thanks: @adrelanos for spotting and reporting
Closes: #256

Failure with stretch:

Processing triggers for initramfs-tools (0.130) ...
update-initramfs: Generating /boot/initrd.img-4.9.0-13-amd64
E: busybox or busybox-static, version 1:1.22.0-17~ or later, is required but not installed
update-initramfs: failed for /boot/initrd.img-4.9.0-13-amd64 with 1.
dpkg: error processing package initramfs-tools (--configure):
 subprocess installed post-installation script returned error exit status 1
[...]
invoke-rc.d: policy-rc.d denied execution of try-restart.
Errors were encountered while processing:
 initramfs-tools
E: Sub-process /usr/bin/dpkg returned an error code (1)