cask/installer: don't call `installed?` if Formula
cask/installer: don't call `installed?` if Formula

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