Create rake task to update playlists on Heroku

  * rake tasks can be scheduled on Heroku, but they cannot be scheduled as
    weekly events
  * Instead, wrap playlist:update:all in a rake task for Heroku that is run
    daily
  * Heroku task will only run on Thursdays for the scheduled time
  * Also, get elements from playlist:update[name] args using hash syntax
Use Gmail for email delivery in production