Fix a false-positive for Rails/RootPathnameMethods on Ruby 2.4 or lower

[r7kamura]

Pathname#glob was added at Ruby 2.5, so it cannot be used on Ruby 2.4 or lower versions.

For example, this false-positive becomes a problem when a gem that uses RuboCop wants to provide code that also supports Ruby 2.4-.

$ ruby --version
ruby 2.4.10p364 (2020-03-31 revision 67879) [x86_64-linux]
$ ruby -r pathname -e "Pathname.new('foo').glob('*')"
-e:1:in `<main>': undefined method `glob' for #<Pathname:foo> (NoMethodError)

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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.