Fix Bundler/OrderedGems cop bug

[colorbox]

Summary

This PR improves Bundler/OrderedGems autocorrect behavior on the
specific situation.
When group includes duplicate gems, and one of duplicate gems is
in the first line of group, and one of them has
offense Bundler/OrderedGems.
The cop replace the gem out of group and the gem in group.

Before

For example, the target Gemfile is like below.

gem 'a'

group :development do
  gem 'b'
  gem 'c'
  gem 'b'
end

Expected

Expected result of rubocop -a --only Bundler/OrderedGems
is like below.

gem 'a'

group :development do
  gem 'b'
  gem 'b'
  gem 'c'
end

Actual

But actual result is like below.

gem 'b'

group :development do
  gem 'a'
  gem 'b'
  gem 'c'
end

---

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 rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

The changelog entry should be moved, as a new release was cut a few days ago.

[colorbox]

Thank you. 🙏 Okay, I'l fix it.

[bbatsov]

Thanks!