Release v0.4.0

Copied over the release script from skaffold to get all new PRs that
have been merged for the CHANGELOG.

Ran dep ensure.

vendor/github.com/google/go-github/github/without_appengine.go

@@ -0,0 +1,19 @@
+// Copyright 2017 The go-github AUTHORS. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !appengine
+
+// This file provides glue for making github work without App Engine.
+
+package github
+
+import (
+	"context"
+	"net/http"
+)
+
+func withContext(ctx context.Context, req *http.Request) *http.Request {
+	return req.WithContext(ctx)
+}