Add ca-certificates to our package Recommends

It's only in "Recommends" because it's only required for all but the esoteric configurations (since you can't "docker pull" from the index without it, but that's about it).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3554 from tianon/ubuntu-dep

Add ca-certificates to our package Recommends