Docker::Event is missing the "Scope" attribute #593

The Moby project's Event schema defines a "Scope" attribute, a string valued either:

1. "local" to indicate a non-Swarm scoped event, such as a container exit.
2. "swarm" to indicate a Swarm scoped event, such as a service update.

Git-blame shows that Scope was added eight years ago, so adding it to Docker::Event shouldn't introduce any incompatibilities.