How to update some application to latest version with homebrew-cask?

I noticed some packages provided by homebrew-cask uses a url of the latest version and a version number of "latest" such as [dropbox](https://github.com/phinze/homebrew-cask/blob/master/Casks/dropbox.rb) and [chrome](https://github.com/phinze/homebrew-cask/blob/master/Casks/google-chrome.rb) . Thus they will never be upgraded with brew upgrade. Is there any simple way to update such packages using homebrew-cask? And if I call brew cask install dropbox again, it shows there already exist some cache of old version, which I feel so dumb.