Add create_or_find_by to save_bang.rb

[MaximeLaurenty]

create_or_find_by will be implemented in Rails 6.0 (rails/rails#31989)
However we've decided to use it before it comes out, and would love it to be in rubocop scope as well.
I guess we're not the first ones to do it, so it could benefit to most.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

CI has failed, so please correct it.
https://circleci.com/gh/rubocop-hq/rubocop-rails/1216

And please add an entry to CHANGELOG.md.

[koic]

Thanks!