Update TimeMachineScheduler for new naming conventions

Per https://github.com/phinze/homebrew-cask/blob/master/CONTRIBUTING.md#naming-casks

This one also seems like it should get the https://github.com/caskroom/homebrew-versions treatment since there is a stable and a beta. What's the process for doing that, one PR here to switch to stable and another PR there to add the beta cask?