Small fixes to Travis configuration
This PR only introduces some very small changes to the Travis build configuration. Namely, it updates the build distro to Ubuntu Focal, and also reinstall the ca-certificates package before building anything to avoid server certificate error when fetching dependencies.
The motivation for creating an entire release for this one small change is that I need the generate_prometheus_targets.sh script to get run as part of a prod deployment to get some changes I made to siteinfo to deploy to production.