Use RuboCop RSpec 2.29

This PR uses RuboCop RSpec 2.29 and suppress the following offenses:

```console
$ bundle exec rubocop
(snip)

Offenses:

spec/rubocop/config_loader_spec.rb:1016:74: C: [Correctable] RSpec/EmptyOutput: Use not_to instead of matching on an empty output.
        expect { expect(configuration_from_file.to_h).to eq(config) }.to output('').to_stderr
                                                                         ^^^^^^^^^^
spec/rubocop/config_loader_spec.rb:1213:18: C: [Correctable] RSpec/EmptyOutput: Use not_to instead of matching on an empty output.
          end.to output('').to_stderr
                 ^^^^^^^^^^
spec/rubocop/config_loader_spec.rb:1249:18: C: [Correctable] RSpec/EmptyOutput: Use not_to instead of matching on an empty output.
          end.to output('').to_stderr
                 ^^^^^^^^^^
spec/rubocop/config_loader_spec.rb:1317:47: C: [Correctable] RSpec/EmptyOutput: Use not_to instead of matching on an empty output.
        expect { configuration_from_file }.to output('').to_stderr
                                              ^^^^^^^^^^

1534 files inspected, 4 offenses detected, 4 offenses autocorrectable
```

Gemfile

@@ -13,7 +13,7 @@ gem 'rake', '~> 13.0'
 gem 'rspec', '~> 3.7'
 gem 'rubocop-performance', '~> 1.21.0'
 gem 'rubocop-rake', '~> 0.6.0'
-gem 'rubocop-rspec', '~> 2.28.0'
+gem 'rubocop-rspec', '~> 2.29.1'
 # Workaround for cc-test-reporter with SimpleCov 0.18.
 # Stop upgrading SimpleCov until the following issue will be resolved.
 # https://github.com/codeclimate/test-reporter/issues/418

spec/rubocop/config_loader_spec.rb

@@ -1013,7 +1013,7 @@ class Loop < Base
               'AllowedPatterns' => []
             }
           )
-        expect { expect(configuration_from_file.to_h).to eq(config) }.to output('').to_stderr
+        expect { expect(configuration_from_file.to_h).to eq(config) }.not_to output.to_stderr
       end
     end
 
@@ -1210,7 +1210,7 @@ class Loop < Base
           expect do
             expect(configuration_from_file['Metrics/MethodLength']
                     .to_set.superset?(expected.to_set)).to be(true)
-          end.to output('').to_stderr
+          end.not_to output.to_stderr
 
           expected = { 'Enabled' => true, # gemtwo/config/default.yml
                        'Max' => 72,              # gemtwo/config/strict.yml
@@ -1246,7 +1246,7 @@ class Loop < Base
           expect do
             expect(configuration_from_file['Metrics/MethodLength']
                     .to_set.superset?(expected.to_set)).to be(true)
-          end.to output('').to_stderr
+          end.not_to output.to_stderr
 
           expected = { 'Enabled' => true, # gemtwo/config/default.yml
                        'Max' => 72,              # gemtwo/config/strict.yml
@@ -1314,7 +1314,7 @@ class Loop < Base
       end
 
       it 'creates the cached file alongside the owning file' do
-        expect { configuration_from_file }.to output('').to_stderr
+        expect { configuration_from_file }.not_to output.to_stderr
         expect(File.exist?(cache_file)).to be true
       end
     end