codecgen: set checkVendor to true from go 1.7+

Updates #164

codec/gen_17.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2012-2015 Ugorji Nwoke. All rights reserved.
+// Use of this source code is governed by a MIT license found in the LICENSE file.
+
+// +build go1.7
+
+package codec
+
+func init() {
+	genCheckVendor = true
+}