Ruby

Install rbenv:

sudo apt install rbenv

Install Ruby:

$ rbenv install 2.4.1
Downloading ruby-2.4.1.tar.bz2...
-> https://cache.ruby-lang.org/pub/ruby/2.4/ruby-2.4.1.tar.bz2
Installing ruby-2.4.1...
Installed ruby-2.4.1 to /home/alex/.rbenv/versions/2.4.1

Start using it:

rbenv global 2.4.1

rbenv install -l does not show latest versions because ruby-build is obsolete. Therefore had to install from github. Then installed latest ruby.

alex@latitude7490:~/Projects/asokolsky.github.io/linux/ > rbenv versions
  system
  2.4.1
* 3.1.1 (set by /home/alex/.rbenv/version)
alex@latitude7490:~/Projects/asokolsky.github.io/linux/ > which ruby
/home/alex/.rbenv/shims/ruby
alex@latitude7490:~/Projects/asokolsky.github.io/linux/ > gem install jekyll bundler