Permit to specify TargetRubyVersion 2.7

[koic]

Parser gem has been started development for Ruby 2.7 (edge Ruby).
whitequark/parser#546

This PR permits Ruby 2.7, the early adapters will be able to try edge Ruby with RuboCop.

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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 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.

[bbatsov]

Thanks!