Commits on Mar 19, 2019

1. [rubocop#5977] Warn for Performance Cops …

   ### Summary
   
   This PR warns for Performance Cops.
   
   The following warning is displayed when the next released rubocop gem is installed.
   
   ```console
   # The following is an example for operation check.
   % rake build
   % gem install pkg/rubocop-0.66.0.gem
   Performance Cops will be removed from RuboCop 0.68. Use
   rubocop-performance gem instead.
   
   Put this in your Gemfile.
   
     gem 'rubocop-performance'
   
   And then execute:
   
     $ bundle install
   
   Put this into your .rubocop.yml.
   
     require: rubocop-performance
   
   More information: https://github.com/rubocop-hq/rubocop-performance
   Successfully installed rubocop-0.66.0
   1 gem installed
   ```
   
   The following warning is displayed if RuboCop is used with Bundler.
   
   ```consle
   % bundle exec rubocop
   [Warn] Performance Cops will be removed from RuboCop 0.68. Use rubocop-performance gem instead.
          https://github.com/rubocop-hq/rubocop/tree/master/manual/migrate_performance_cops.md
   ```
   
   The above warning will disappear when rubocop-performance gem is written to Gemfile.
   The rubocop-performance implementation takes precedence when rubocop-performance is
   written in Gemfile.
   
   And I prepared a simple migration manual (migrate_performance_cops.md)
   because I think that long warning sentences are not preferred.
   I think that referring to the link is good when runnning rubocop command.
   
   ### Other Infromation
   
   At first I tried to remove performance cops from RuboCop Core and
   add rubocop-performance gem as a dependency because of fallback.
   
   ```diff
   % git diff
   diff --git a/rubocop.gemspec b/rubocop.gemspec
   index bbbcca33f..09882b063 100644
   --- a/rubocop.gemspec
   +++ b/rubocop.gemspec
   @@ -38,6 +38,7 @@ Gem::Specification.new do |s|
      s.add_runtime_dependency('parser', '>= 2.5', '!= 2.5.1.1')
      s.add_runtime_dependency('psych', '>= 3.1.0')
      s.add_runtime_dependency('rainbow', '>= 2.2.2', '< 4.0')
   +  s.add_runtime_dependency('rubocop-performance')
      s.add_runtime_dependency('ruby-progressbar', '~> 1.7')
      s.add_runtime_dependency('unicode-display_width', '>= 1.4.0', '< 1.6')
   ```
   
   However, this cannot be resolved because it is cyclic.
   
   ```console
   % bundle install
   Fetching gem metadata from https://rubygems.org/........
   Fetching gem metadata from https://rubygems.org/.
   Resolving dependencies...
   Your bundle requires gems that depend on each other, creating an infinite loop.
   Please remove either gem 'rubocop' or gem 'rubocop-performance' and try again.
   ```
   
   So I'd like to display a warning before removing performance cops.
   
   I'd like to mitigate the impact on users this way.
   
   The next step is to remove performance cops and done rubocop#5977.

   

   koic committed Mar 19, 2019
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