Fix false positive in Style/RedundantSelf cop with nested self access

[marcotc]

This PR fixes the following false positive:

a = self.a || b

which currently returns:

file.rb:1:5: C: Style/RedundantSelf: Redundant self detected.
a = self.a || b
    ^^^^^^

This is a false positive, as self.a cannot be changed to a while keeping the same semantics. Changing it to a would effectively make the left-hand side of this or statement always nil, as a has just been declared and still doesn't have a value assigned.

This happens because the logic on redundant_self.rb does not check for nested statements (in the example, self.a is nested node inside an or node).
This PR adds an ancestor search, starting from self.a, to try to find if any of its ancestors are associated with a local variable to name a. The changes introduced work for any level of nesting.

---

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]

The changes look good, but your branch has to be rebased on top of the current master branch due to merge conflicts.

[marcotc]

Thank you, @bbatsov, a simple rebase did the trick. Let me know if there's any additional feedback!

[bbatsov]

@marcotc I totally dropped the ball on you. Sorry! Can you rebase again and move the changelog entry to the unreleased section?

[marcotc]

Sorry for not rebasing earlier, @bbatsov, I was on vacation recently. Thank you very much on merging it!

Also, huge congratulations on shipping 1.0!