gitignore: don't vendor rubocop-ast.
gitignore: don't vendor rubocop-ast.

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