Add rubocop-performance gem and config to fix deprecation message (#430)

* Add rubocop-performance gem and config to fix deprecation message

* ehhh apparently the order of the gemfile matters?

Gemfile:14:3: C: Bundler/OrderedGems: Gems should be sorted in an alphabetical order within their section of the Gemfile. Gem rubocop-github should appear before rubocop-performance.
  gem "rubocop-github"
  ^^^^^^^^^^^^^^^^^^^^

.rubocop.yml

@@ -1,3 +1,4 @@
 inherit_gem:
   rubocop-github:
     - config/default.yml
+require: rubocop-performance

Gemfile

@@ -11,6 +11,7 @@ group :test do
   gem "rspec"
   gem "rubocop", "< 0.68"
   gem "rubocop-github"
+  gem "rubocop-performance"
   gem "term-ansicolor"
   gem "tins"
 end