Add new Rails/RedundantTravelBack cop

[koic]

This PR adds new Rails/RedundantTravelBack cop. It checks for redundant travel_back.
Since Rails 5.2, travel_back automatically called at the end of the test.

# bad
def teardown
  do_something
  travel_back
end

# good
def teardown
  do_something
end

# bad
after do
  do_something
  travel_back
end

# good
after do
  do_something
end

It depends on minimum_target_rails_version, so it's packaged in this RuboCop Rails instead of RuboCop RSpec.

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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 (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.