ci(travis): Bump Ubuntu base image to Bionic 18.04
ci(travis): Bump Ubuntu base image to Bionic 18.04

We don't even build PPA packages for Xenial, why test on it? Given that
there are two newer Ubuntu LTS releases to work from and that we
explicitly support, I think we can stop testing anything older than
what is in Bionic.

alerque/sile,alerque/sile,alerque/sile,alerque/sile