hostile alien CAPTCHA objective added

So this is my personal preference for the objective as I think it fits well with it acting like a CAPTCHA and at the same time setting an artificial but understandable reason why you'd used this problem for it (the troll part was not random).  However it is a quite out there objective, don't know whether want to keep it more direct and to the point.  Alternative is a more mundane explanation: https://github.com/animoto/platform_engineer_interview/compare/mundane_objective 

The lines after the objective are the same in each.  +1 this pull request or the mundane_objective one.