Suppress a RuboCop's offense

This PR suppresses the following RuboCop's offense:

```console
$ bundle exec rubocop
(snip)

Offenses:

rubocop-rake.gemspec:34:8: C: [Correctable] Gemspec/AddRuntimeDependency: Use add_dependency instead of add_runtime_dependency.
  spec.add_runtime_dependency 'rubocop', '~> 1.0'
       ^^^^^^^^^^^^^^^^^^^^^^

25 files inspected, 1 offense detected, 1 offense autocorrectable
```

rubocop-rake.gemspec

@@ -31,5 +31,5 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_runtime_dependency 'rubocop', '~> 1.0'
+  spec.add_dependency 'rubocop', '~> 1.0'
 end