Suppress RuboCop offenses

This commit suppresses the following new RuboCop offenses:

```console
$ bundle exec rubocop
(snip)

rubocop-rails.gemspec:34:5: C: [Correctable] Gemspec/AddRuntimeDependency:
Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'activesupport', '>= 4.2.0'
    ^^^^^^^^^^^^^^^^^^^^^^
rubocop-rails.gemspec:37:5: C: [Correctable] Gemspec/AddRuntimeDependency:
Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'rack', '>= 1.1'
    ^^^^^^^^^^^^^^^^^^^^^^
rubocop-rails.gemspec:38:5: C: [Correctable] Gemspec/AddRuntimeDependency:
Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'rubocop', '>= 1.33.0', '< 2.0'
    ^^^^^^^^^^^^^^^^^^^^^^
rubocop-rails.gemspec:39:5: C: [Correctable] Gemspec/AddRuntimeDependency:
Use add_dependency instead of add_runtime_dependency.
  s.add_runtime_dependency 'rubocop-ast', '>= 1.31.1', '< 2.0'
    ^^^^^^^^^^^^^^^^^^^^^^

294 files inspected, 4 offenses detected, 4 offenses autocorrectable
```

Author: koic

rubocop-rails.gemspec

@@ -31,10 +31,10 @@ Gem::Specification.new do |s|
     'rubygems_mfa_required' => 'true'
   }
 
-  s.add_runtime_dependency 'activesupport', '>= 4.2.0'
+  s.add_dependency 'activesupport', '>= 4.2.0'
   # Rack::Utils::SYMBOL_TO_STATUS_CODE, which is used by HttpStatus cop, was
   # introduced in rack 1.1
-  s.add_runtime_dependency 'rack', '>= 1.1'
-  s.add_runtime_dependency 'rubocop', '>= 1.33.0', '< 2.0'
-  s.add_runtime_dependency 'rubocop-ast', '>= 1.31.1', '< 2.0'
+  s.add_dependency 'rack', '>= 1.1'
+  s.add_dependency 'rubocop', '>= 1.33.0', '< 2.0'
+  s.add_dependency 'rubocop-ast', '>= 1.31.1', '< 2.0'
 end