Added EBS snapshot changes

@mmell 
https://trello.com/c/XDCFeDX2/75-cnc-should-perform-ebs-snapshots
@cjawale the method signature of `prune_snapshot_by_instance_ids(region, instance_ids, age_in_days)` has arguments swapped around from `delete_snapshots(volume_ids, age_in_days, region) `. Please make every method as consistent as possible across the entire gem.