Fix Rubocop performance deprecation warning

[kodyclemens]

Fixes #5795

• gem 'rubocop-performance' added to Gemfile
• require: rubocop-performance added to .rubocop.yml
• PR is descriptively titled 📑 and links the original issue above 🔗
• [] tests pass -- look for a green checkbox ✔️ a few minutes after opening your PR -- or run tests locally with rake test
• code is in uniquely-named feature branch and has no merge conflicts 📁
• screenshots/GIFs are attached 📎 in case of UI updation
• ask @publiclab/reviewers for help, in a comment below

Codeclimate test currently not passing. I am unfamiliar with Codeclimate but the Travis CI is passing.

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[cesswairimu]

Hi @kodyclemens 👋 and welcome to publiclab/plots2 🎈 thanks for working on this issue. I saw the codeclimate issue raised in this PR and after a little research I realized that there is no support for this yet I should have looked into this first codeclimate/codeclimate-rubocop#165. However I think we should go back to version 0.67 which still have performance cop build inside the rubocop gem. But would like to hear @gauravano and @jywarren thoughts on this. Thanks