Update RuboCop config for 1.8 compatibility

[Knack]

RuboCop has recently released version 1.8, which triggers new warnings with our current configuration:

• The Naming/PredicateName cop has been renamed to Naming/PredicatePrefix.
• The rubocop-rails, rubocop-performance, and rubocop-rspec extensions now support plugins. You should specify plugins: ... instead of require: ... in your configuration files.
  See: https://docs.rubocop.org/rubocop/plugin_migration_guide.html

To address these, I have created a rubocop-1.8 branch containing the necessary adjustments.

This pull request will not be merged immediately; we will wait until our main applications have upgraded to RuboCop 1.8.
If you want to use RuboCop 1.8 in your project before then, update the inherit_from attribute in your .rubocop.yml to point to the rubocop-1.8 branch:

inherit_from:
  - https://raw.githubusercontent.com/cookpad/global-style-guides/rubocop-1.8/.rubocop.yml