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. 
The Rails version?

The current bundle gemfile may have been changed since the engine cart test app was generated. Or, as Blacklight is currently doing, the current bundle gemfile may use an ENV variable to specify rails version, and the ENV may have changed. 

This is meant to snapshot the Rails version that was used when the test app was generated. But yes, at time of execution, it should be the same version of Rails that would be chosen by the current Gemfile context, yep, that's the idea!