Use app definition for task in same version.

With https://github.com/allegro/marathon-consul/pull/145 and https://github.com/allegro/marathon-consul/issues/164 we put a plaster on Marathon app task differences. We should obtain app definition for task in version defined in task. We can use `/v2/apps/{app_id}/versions/{version}` endpoint to get this information. What's more we can cache app definitions and to reduce calls to marathon. Only drawback of this solution is versions could not be available since it could be configured to keep last `n` entries. We need to think what we should do in that circumstances.