Add a cask for Apptivate.app, 2.1 (12)

This diff creates a new cask for Homebrew Cask for the app called
Apptivate, which can be found at http://www.apptivateapp.com. I followed
the instructions on contributing found on Homebrew Cask to create
this.
Test plan: install using `brew cask install apptivate`, launch
Apptivate using `open -a "Apptivate"`, and check the version and if
everything works.
Update to latest versioning scheme

As per @NanoXD's suggestion, I've changed the version and the SHA sum to
`latest_version` and `no_checksum`.
Merge pull request #1668 from gnarmis/apptivate

Add a cask for Apptivate.app, 2.1 (12)