Enable NewCops for RuboCop development

Pending cops will be enabled by default in RuboCop 1.0.

This PR enables `NewCops` for RuboCop development. It will be
a verification that enables pending cops.

And this PR suppresses the following offense.

```console
% cd path/to/rubocop
% bundle exec rake
(snip)

lib/rubocop/config_loader_resolver.rb:184:7: C:
Style/HashTransformValues: Prefer transform_values over Hash[_.map {...}].
      Hash[config.map { |cop, params| [cop, yield(params)] }]
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1144 files inspected, 1 offense detected
```

.rubocop.yml

@@ -7,6 +7,7 @@ require:
   - rubocop-rspec
 
 AllCops:
+  NewCops: enable
   Exclude:
     - 'vendor/**/*'
     - 'spec/fixtures/**/*'

lib/rubocop/config.rb

@@ -47,7 +47,7 @@ def check
     end
 
     def_delegators :@hash, :[], :[]=, :delete, :each, :key?, :keys, :each_key,
-                   :map, :merge, :to_h, :to_hash
+                   :map, :merge, :to_h, :to_hash, :transform_values
     def_delegators :@validator, :validate, :target_ruby_version
 
     def to_s

lib/rubocop/config_loader_resolver.rb

@@ -181,7 +181,7 @@ def handle_disabled_by_default(config, new_default_configuration)
     end
 
     def transform(config)
-      Hash[config.map { |cop, params| [cop, yield(params)] }]
+      config.transform_values { |params| yield(params) }
     end
 
     def gem_config_path(gem_name, relative_config_path)