Suppress deprecation warnings of Psych.safe_load args in Ruby 2.6

Follow up of #6395 (comment).

The interface of `Psych.safe_load` will change from Ruby 2.6.
ruby/ruby@1c92766

This PR suppresses the following wargnins.

```console
cd /path/to/rubocop/repo
% ruby -v
ruby 2.6.0dev (2018-10-21 trunk 65252) [x86_64-darwin17]
bundle exec rspec

(snip)

warning: Passing whitelist_classes with the 2nd argument of
Psych.safe_load is deprecated. Use keyword argument like
Psych.safe_load(yaml, whitelist_classes: ...) instead.
warning: Passing whitelist_symbols with the 3rd argument of
Psych.safe_load is deprecated. Use keyword argument like
Psych.safe_load(yaml, whitelist_symbols: ...) instead.
warning: Passing aliases with the 4th argument of
Psych.safe_load is deprecated. Use keyword argument like
Psych.safe_load(yaml, aliases: ...) instead.
warning: Passing filename with the 5th argument of
Psych.safe_load is deprecated. Use keyword argument like
Psych.safe_load(yaml, filename: ...) instead.
```

lib/rubocop/config_loader.rb

@@ -181,6 +181,14 @@ def yaml_safe_load(yaml_code, filename)
         if defined?(SafeYAML) && SafeYAML.respond_to?(:load)
           SafeYAML.load(yaml_code, filename,
                         whitelisted_tags: %w[!ruby/regexp])
+        elsif RUBY_VERSION >= '2.6'
+          YAML.safe_load(
+            yaml_code,
+            whitelist_classes: [Regexp, Symbol],
+            whitelist_symbols: [],
+            aliases: false,
+            filename: filename
+          )
         else
           YAML.safe_load(yaml_code, [Regexp, Symbol], [], false, filename)
         end