Ruby: Upgrade from 2.5.3 to 2.5.7

[DeeDeeG]

Context

• Update Ruby from 2.5.3 to 2.5.7 (security and other bugfixes, etc.)
• Incidentally, the newer Ruby 2.5.7-slim Docker image is based on Debian 10 "Buster" by default, as opposed to Debian 9 "Stretch" for the older Ruby 2.5.3-slim Docker image.

Release Notes

Here are the release notes for Ruby versions from 2.5.4 to 2.5.7:

• 2.5.4, 2.5.5, 2.5.6, 2.5.7

Summary of Changes

• Update .ruby-version
• Update Dockerfile
• Update Gemfile and Gemfile.lock
• Update README.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)

[DeeDeeG]

I tested this manually in a local Docker container, and it seemed fine.

CI is broken for unrelated reasons.

[DeeDeeG]

Planning to merge today if no conflicts with the other two PRs.

[DeeDeeG]

I added a small workaround for failing tests to this PR.

It involves setting an environment as follows: OPENSSL_CONF=/etc/ssl/. See this StackOverflow question for details: https://stackoverflow.com/questions/53355217/genymotion-throws-libssl-conf-so-cannot-open-shared-object-file-no-such-file-o

I assume this has to do with the base image for ruby:2.5.7-slim being Debian 10 "Buster", as opposed to Debian 9 "Stretch," which is the base image for the older ruby:2.5.3-slim.