Add rubocop-performance dev dependency

[vtnate]

Resolves #[issue number here]

Pull Request Description

This is required by rubycop config. Previous versions inherited rubocop sub-dependencies through extension-gem (IIRC) that we now have to explicitly provide in each gem.

Checklist (Delete lines that don't apply)

• Unit tests have been added or updated
• Documentation has been modified appropriately
• All ci tests pass (green)
• An issue has been created (which will be used for the changelog)
• This branch is up-to-date with develop

[coveralls]

Pull Request Test Coverage Report for Build 18291925333

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 87.541%

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Totals
Change from base Build 18291547057:	0.0%
Covered Lines:	794
Relevant Lines:	907

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💛 - Coveralls