Include a Cop to enforce refute(actual) over assert_equal(false)

[duduribeiro]

This commit adds a Cop that checks some usage of
assert_equal(false, something) and enforces the usage of
refute(something) instead.

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

---

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]

Can you add the changelog entry about this one?

[duduribeiro]

@koic sorry for the late response. I did add the entry into the changelog. thanks so much

[duduribeiro]

@bbatsov, @koic I did update the branch with the changes suggested here. Thanks

[bbatsov]

Thanks!