[master] 1b606ff94 ci: Increase parallelism in Travis CI builds

[Dridi Boukelmoune]

> We saved 10 minutes per build (a couple minutes per job). But I think
> most of the time is wasted waiting for and booting VMs instead of
> container builds...

I was a bit behind on travis build environments and got mentally stuck
to the old "historical" container setup:

https://docs.travis-ci.com/user/reference/overview/#deprecated-virtualization-environments

Too my surprise nowadays x86_64 systems are the only ones that don't
containerize on Travis CI and I'm wondering whether we shouldn't
instead use all the other architectures, especially those that we
don't get with Circle CI that I occasionally test using Fedora's
infrastructure:

https://docs.travis-ci.com/user/reference/overview/#which-one-do-i-use

Except for the macos and sanitizer jobs, we could run "normal" jobs in
aarch64, ppc64le and s390x containers with a faster spin up, but
possibly slower execution speed (at least that's how aarch64 looks
today).

Thoughts?