Fix rubocop warnings

[k0kubun]

This PR fixes the following warnings:

rubocop-performance extension supports plugin, specify `plugins: rubocop-performance` instead of `require: rubocop-performance` in /home/k0kubun/src/github.com/ruby/ruby-bench/benchmarks/rubocop/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.

rubocop-rails extension supports plugin, specify `plugins: rubocop-rails` instead of `require: rubocop-rails` in /home/k0kubun/src/github.com/ruby/ruby-bench/benchmarks/rubocop/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.