Update to Ruby 2.5.1

[octopusinvitro]

WARNING: ruby-2.2.1 is past its end of life and is now unsupported.
It no longer receives bug fixes or critical security updates.

Our current Ruby version is 2.2.1. Switch the Ruby version to 2.5.1

To get started

• Claim this issue by clicking the "Assignees" cog to the right and assigning yourself
• Check out our contributing guides
• You can pair with a coach

Steps for this issue:

• Install Ruby version 2.5.1 on your local system. How to do this depends on the Ruby version manager that you use. (this is an example for the rbenv manager)
• Set your Ruby version to 2.5.1. This again depends on your Ruby version manager.
• Change the Ruby version in the .ruby-version file of this project.
• Install the bundler gem for that version: gem install bundler.
• Run bundle install and check that the tests if any and the app work correctly.
• You may or may not have to upgrade some gems. If you have to, do it one at a time, like this:
  • find a conservative version of the gem that works and update the Gemfile
  • run bundle update GEMNAME
  • run the tests if any, they should be green
  • Run the app, it should work
  • commit!
• When you are finished, push your branch and submit a PR with your changes.

[Aliciawyse]

Hi! I'd like to do this. Creating a fork now. FYI, I can't make myself the assignee. Not sure why.

[Aliciawyse]

Here's the PR #403