packer/rpc: register map[string]string since we transfer that

packer/rpc/init.go

@@ -4,6 +4,7 @@ import "encoding/gob"
 
 func init() {
 	gob.Register(new(map[string]interface{}))
+	gob.Register(new(map[string]string))
 	gob.Register(make([]interface{}, 0))
 	gob.Register(new(BasicError))
 }