[APPS-933] - As a ZAT user, I want "ZAT new" to give me the current version in my default manifest.json

:koala: :+1: 

This PR replaces the hard-coded version output to the default manifest via `zat new` with whatever the value of `ZendeskAppsSupport::AppVersion::CURRENT`... currently is. 

/cc @zendesk/quokka