Update codegen to 2018 edition
diff --git a/codegen/src/json.rs b/codegen/src/json.rs
index d870a64..66803ee 100644
--- a/codegen/src/json.rs
+++ b/codegen/src/json.rs
@@ -1,5 +1,7 @@
 use crate::types;
 
+use serde::{Serialize, Deserialize};
+
 use std::collections::BTreeMap;
 use std::fs::{self, File};
 use std::io::prelude::*;