Suppress an offense for rails/presence_spec.rb

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

```console
% 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

spec/rubocop/cop/rails/presence_spec.rb

@@ -67,7 +67,7 @@
 
   it_behaves_like 'offense', 'a if a.present?', 'a.presence', 1, 1
   it_behaves_like 'offense', 'a unless a.blank?', 'a.presence', 1, 1
-  it_behaves_like 'offense', <<~RUBY.chomp, <<~FIXED.chomp, 1, 7 # rubocop:disable Metrics/LineLength
+  it_behaves_like 'offense', <<~RUBY.chomp, <<~FIXED.chomp, 1, 7
     if [1, 2, 3].map { |num| num + 1 }
                 .map { |num| num + 2 }
                 .present?