Provide instructions on how to use RVM or rbenv to install Ruby version not provided by the OS

This issue regards to the [Setup episode](https://carpentries.github.io/lesson-example/setup.html), the section of installing `Ruby` for Linux user.

When installing by `apt-get`, the default Ruby version is `2.5.1` (after `apt-get update`), which may be too old for some users. An alternative way of installation is to use `RVM`, e.g. mentioned in [this tutorial](https://gorails.com/setup/ubuntu/18.04).