Show Ruby version of the current dir for rubocop -V

[Earlopain]

The rules are complex, even within the same project different ruby versions can be used for analysis. This makes rubocop -V output the ruby version of the current working directory.

The first commit is a bit of cleanup. It was more difficult than necessary to understand where the version method is being used and how. Most callers simply want RuboCop::Version::STRING, i've updated these places. I wanted to just remove the debug option since it is marked private but standardrb uses it and other places use RuboCop::Version.version as well anyways, regardless of intended visibility. Instead I've left a note, so that if it does get changed in the future, it can be more coordinated.

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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.

[bbatsov]

even within the same project different ruby versions can be used for analysis.

It never crossed my mind someone might want to do something like this, but I guess in makes sense in some mono-repo setups when you have multiple projects.

[Earlopain]

Yeah, monorepos are a possible use-case. Rails uses rubocop-md and analyses code with a more recent version that what they themselves support so they can use more modern syntax in docs.