Suppress an offense for rails/presence_spec.rb

[koic]

This PR suppresses the following offense for rails/presence_spec.rb.

% bundle exec rake
(snip)

Offenses:

spec/rubocop/cop/rails/presence_spec.rb:70:66: W:
Lint/UnneededCopDisableDirective: Unnecessary disabling of
Metrics/LineLength.
  it_behaves_like 'offense', <<~RUBY.chomp, <<~FIXED.chomp, 1, 7
  # rubocop:disable Metrics/LineLength
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

126 files inspected, 1 offense detected
RuboCop failed!

https://circleci.com/gh/rubocop-hq/rubocop-rails/1721

---

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• 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.