Add Cask Xamarin, replacing older Xamarin Casks

These older scraped Casks are XamarinAndroid, XamarinIos, and XamarinStudio. I have submitted a pull request to have these Casks added to homebrew-versions, as they have been replaced by Xamarin's new installer, included in this commit. See #5099 for comments by collaborators.

See pull request caskroom/homebrew-versions#307, which adds the scraped versions of XamarinAndroid, XamarinIos, and XamarinStudio to the homebrew-versions Cask repository.