Update Ruby from 2.5.7 to 2.5.8

[DeeDeeG]

Context

• Update Ruby. See the release notes:
  • https://www.ruby-lang.org/en/news/2020/03/31/ruby-2-5-8-released/

Summary of Changes

• Update Ruby version number in .ruby_version, Dockerfile, Gemfile and Gemfile.lock, and REAMDE.md

Checklist

• [ ] Tested Mobile Responsiveness
• [ ] Added Unit Tests
• [ ] CI Passes
• [ ] Deploys to Heroku on test Correctly (Maintainers will handle)
• [ ] Added Documentation (Service and Code when required)