change dep to go module

vendor/github.com/gin-gonic/gin/json/json.go

@@ -0,0 +1,15 @@
+// Copyright 2017 Bo-Yi Wu.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+// +build !jsoniter
+
+package json
+
+import "encoding/json"
+
+var (
+	Marshal       = json.Marshal
+	MarshalIndent = json.MarshalIndent
+	NewDecoder    = json.NewDecoder
+)

vendor/github.com/gin-gonic/gin/json/jsoniter.go

@@ -0,0 +1,16 @@
+// Copyright 2017 Bo-Yi Wu.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+// +build jsoniter
+
+package json
+
+import "github.com/json-iterator/go"
+
+var (
+	json          = jsoniter.ConfigCompatibleWithStandardLibrary
+	Marshal       = json.Marshal
+	MarshalIndent = json.MarshalIndent
+	NewDecoder    = json.NewDecoder
+)