Switch TimeMachineScheduler cask to stable

Moving beta to https://github.com/caskroom/homebrew-versions, will mention this PR over there so they get cross-linked.

This was a preference pane within an installer app within a DMG within a zip. Could this be the turducken of casks?

From the readme inside the DMG:
>The enclosed installer has to options:
>Install installs or updates TimeMachineScheduler 3 in /Library/PreferencePanes and removes all unused components of previous versions. The settings of TimeMachineScheduler 2 will be preserved.
>You have to enter the admin password twice: during the installation process and once at the first launch.

>Uninstall removes all components of TimeMachineScheduler.