Fix a false positive for Style/RedundantParentheses

[koic]

This PR fixes a false positive for Style/RedundantParentheses when regexp literal attempts to match against a parenthesized condition.

The presence or absence of parentheses here changes the meaning of the syntax, so the parentheses are not redundant:

$ ruby-parse -e '/regexp/ =~ (foo || bar)'
(match-with-lvasgn
  (regexp
    (str "regexp")
    (regopt))
  (begin
    (or
      (send nil :foo)
      (send nil :bar))))

$ ruby-parse -e '/regexp/ =~ foo || bar'
(or
  (match-with-lvasgn
    (regexp
      (str "regexp")
      (regopt))
    (send nil :foo))
  (send nil :bar))

---

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]

The change is good, but I think it'd be also good to expand a bit the description of the cop and its examples, as both are too basic right now.