[APPS-435] Fix requiredProperties bug

This fixes an issue where requiredProperties could be changed by another instance of the app.

JIRA: https://zendesk.atlassian.net/browse/APPS-435

The same change will have to made to [Magento](https://github.com/zendesk/magento_app/blob/master/app.js#L35) and [Shopify](https://github.com/zendesk/shopify_app/blob/master/app.js#L45)

/cc @jwswj @s-vizzari