Adapter#listVersions

_ember-cli-deploy_ project implements adapter's interface. I think that in future we may require improvements of this feature like displaying versions sorted by last date it was activated or something like that. To my mind, the logic of calculating such additional meta information ( e.g. lastTimeActivated ) should be incapsulated in adapter itself and be exposed via it's interface. Converting this to object gives us more room to include additional fields, which later may be exposed by listVersions method and consumed in ember-cli-deploy. If you think it's not a good reason to do this, I will change. Thanks.