Allow Sandbox to work with multiple Ruby versions

[kennyadsl]

At the moment our sandbox is using Rails 6, so it requires Ruby >= 2.5 and this could create issues to users using Ruby 2.4, which has not yet reached EOL (which will happen on 2020-03-31).

Ref: #3411

Describe the solution you'd like

We should try to create a Rails 5.2 application rather than a Rails 6 one, if Ruby < 2.5.

Describe alternatives you've considered

Alternatively, we should document requirements better in our README to help new users with the confusion here.