Change Rails/RootPathnameMethods to detect offenses on Dir.[]

[r7kamura]

Added support for Dir.[].

One of the most commonly used example of this pattern is the following code, generated by rspec-rails gem:

# bad
Dir[Rails.root.join('spec/support/**/*.rb')].each { |f| require f }

# good
Rails.root.glob('spec/support/**/*.rb').each { |f| require f }

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

This looks good to me. Can you rebase with the latest master branch?