[INFRA] Migrate to container-based Travis CI

[aalexandrov]

Emma is currently using a legacy-based Travis configuration.

It seems that migrating to container-based infrastructure has some benefits.

I think it is worth trying this out to check whether there is a speedup from the ~24mins of build time we see at the moment.

Instead of sudo apt-get install -y pv, we will have to install the pv package with the addons directive.