Automatically manage minor versions of Ruby

[dvalfre]

Similar to how #208 was handled, we want to manage minor versions of Ruby to avoid having to do work like PR #227 every time a new minor version comes out.
The referred PR shows where the versions are being handled in the cookbooks. The file at /home/deploy/.cache/ruby-install/ruby/versions.txt lists the available versions. Said file, though, is only updated if ruby-install is run, thus posing a challenge on how to update versions without actually installing them. Alternatively, we can explore the use of ruby-install -L to install the latest version regardless.