Revert "Use AppVersion::CURRENT to set default manifest version"

This reverts commit ca567bc21abb781b66071dc25163eec6f97959d5.
Add deprecation warning for validation
Refactor to log warnings collated during validation
Update zendesk_apps_support
Merge pull request #23 from zendesk/stef/APPS_739_validate_versioning

[APPS-739] - As a ZAT user, I want ZAT to ensure my app targets a valid version of the framework