Update README ruby version 2.3->2.4

What does this MR do?

Updates the version of Ruby specified in the project README from 2.3 to 2.4

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides