gitignore: add rubocop-performance.
gitignore: add rubocop-performance.

mahori/brew,EricFromCanada/brew,mahori/brew,MikeMcQuaid/brew,EricFromCanada/brew,nandub/brew,nandub/brew,mahori/brew,sjackman/homebrew,sjackman/homebrew,sjackman/homebrew,vitorgalvao/brew,claui/brew,claui/brew,claui/brew,konqui/brew,Homebrew/brew,EricFromCanada/brew,reitermarkus/brew,nandub/brew,JCount/brew,nandub/brew,reitermarkus/brew,vitorgalvao/brew,JCount/brew,JCount/brew,konqui/brew,JCount/brew,Homebrew/brew,mahori/brew,MikeMcQuaid/brew,vitorgalvao/brew,reitermarkus/brew,EricFromCanada/brew,Homebrew/brew,MikeMcQuaid/brew,reitermarkus/brew,konqui/brew,sjackman/homebrew,claui/brew,MikeMcQuaid/brew,konqui/brew,vitorgalvao/brew,Homebrew/brew