Defines the show action of CrewController
Defines the CrewController's update action

In this commit the index.json.rabl and show.json.rabl have been renamed
(removing .json) to allow access in url without the need for .json
extension
Uses 'rack-cors' Gem for cross-origin resource sharing
Skips verification of authenticity token since the requests are coming from outside of this Rails app
Adds validations to the Crew model