Upgrade gin-gonic to latest master, fix compatibility issues

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

@@ -0,0 +1,11 @@
+// +build appengine
+
+// Copyright 2017 Manu Martinez-Almeida.  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
+
+func init() {
+	defaultAppEngine = true
+}