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

[rogerkuou]

This issue regards to the Setup episode, 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.

[SJaffa]

Currently using apt-get gives Ruby 2.7.0, but when I then make serve I get an error because the Gemfile specifies Ruby>=2.7.1, so it is now necessary to either use an alternative installation method or change the Gemfile (which I don't know enough about Ruby to mess with)