Switch to container-based Travis run

[frangio]

Travis jobs have been intermittently failing because of some issues on their side with sudo: required builds. This configuration line apparently means that the job will run in a virtual machine, as opposed to sudo: false which means the job will run in a container.

It doesn't look like we will need sudo, and the container-based run would be apparently slightly faster to boot. Let's see how this works.

(The first run is expected to be slower because it rebuilds the cache after a configuration change, IIRC.)

[nventuro]

I find it weird that their docs say this for sudo: true:

This has the advantage that no state persists between builds, offering a clean slate and making sure that your tests run in an environment built from scratch

, and this for sudo: false:

The container contents are a pristine copy of the Docker image, as guaranteed by Docker itself

Anyway, it seems either will suit us, and the build times seem to be similar, so I'll merge and we'll see how it goes over the next few days.