Suppress deprecation warnings of Psych.safe_load args in Ruby 2.6

[koic]

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.

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.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

👍