store environment details in the .generated_engine_cart

Looking towards a future feature that can notice a 'stale' generated app (for a different ruby or rails), and clean it and regenerate it automatically.  

But in the meantime, it's still potentially useful to store some contextual information in the .generated_engine_cart file that already exists, for possible debugging purposes. At any rate, doesn't hurt. 
Won't this be the same as the version specified in the bundle gemfile?