add vendor & update README.md

vendor/github.com/gin-gonic/gin/response_writer_1.7.go

@@ -0,0 +1,12 @@
+// +build !go1.8
+
+// Copyright 2018 Gin Core Team.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package gin
+
+// ResponseWriter ...
+type ResponseWriter interface {
+	responseWriterBase
+}