Update cask/installer.rb: check dependencies before download
Update cask/installer.rb: check dependencies before download

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