packer/rpc: Register []interface{} so arrays can be sent over wire

packer/rpc/init.go

@@ -4,5 +4,6 @@ import "encoding/gob"
 
 func init() {
 	gob.Register(new(map[string]interface{}))
+	gob.Register(make([]interface{}, 0))
 	gob.Register(new(BasicError))
 }