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

This commit expands the Netlify documentation page to include a
description of how to manually deploy a built Gutenberg site from the
command line.

This is extremely minor, but the initial `netlify.toml` example confused
me a bit because I didn't realize that it was using a *netlify*
variable; I thought I needed to replace something with a variable from
my own configuration.  I made a very minor edit to the docs to clarify.