Alter BookstoreVersionHandler response

[mpacer]

This arose in the context of #130.

This will change the validation logic available, and so would be relevant for nteract_on_jupyter as it uses this information as part of it's startup page.

@stormpython @captainsafia

[mpacer]

Initially, this was a statement to remove the "bookstore": True field from the response, but in conversation with @MSeal and @willingc it became clear that we might want to reconsider the structure of this endpoint altogether.

[stormpython]

@mpacer thats fine. I will open an issue and PR in the nteract repo to address this change.