If not specified, Travis use a system with ruby, which we don't need. We can use the "minimal" one instead.

Closes #530

Closes #636

Fix #613

Fix #616. Add brief documentation for how to use it.

This reverts commit 5fd7bf7e61.

Apparently it is not just no longer necessary to use classic
confinement, but actually impossible, i.e. snap emits an error that the
zola snap is not compatible with `--classic`.

Closes #576

690b3f9 updated the required version in Cargo.lock and in the CI config files, but not in the installation instructions.

Closes #537

Closes #533

If you don't install with classic confinement, you'll get permission errors trying to create sites with `zola init`.