Update Popcorn-Time to fix version & url

  * fix version
  * prefer HTTPS to HTTP
  * refactor to 
    * [facilitate future updates][ref0]
    * [follow standard cask structure][ref1]

  [ref0]: https://github.com/caskroom/homebrew-cask/pull/4910
  [ref1]: https://github.com/caskroom/homebrew-cask/blob/master/CONTRIBUTING.md#the-brew-cask-create-command