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.