use travis "minimal" vm in the documentation

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

v.0.6.0

Allow default base-path command line option to be set for building and serving.

Close #633

Split sitemap when it is getting too big

To ensure we will get the right `content`
directory.
Fix #629

Fix minor documentation quirks in templates/overview

Fix #613

Add maxdeviant.com to examples

adding back default in rebuild

Fix #620

use nix-shell to build on nixos

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

fix minor typo in doc template

Handle csv parsing error when encountering rows with different lengths

Closes #607

Revert "Update snap installation instructions"

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`.

Fix format mistake