Update the Microsoft Oauth2 lib.

vendor/golang.org/x/oauth2/LICENSE

-Copyright (c) 2009 The oauth2 Authors. All rights reserved.
+Copyright (c) 2009 The Go Authors. All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are

vendor/golang.org/x/oauth2/microsoft/microsoft.go

@@ -14,3 +14,18 @@ var LiveConnectEndpoint = oauth2.Endpoint{
 	AuthURL:  "https://login.live.com/oauth20_authorize.srf",
 	TokenURL: "https://login.live.com/oauth20_token.srf",
 }
+
+// AzureADEndpoint returns a new oauth2.Endpoint for the given tenant at Azure Active Directory.
+// If tenant is empty, it uses the tenant called `common`.
+//
+// For more information see:
+// https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-v2-protocols#endpoints
+func AzureADEndpoint(tenant string) oauth2.Endpoint {
+	if tenant == "" {
+		tenant = "common"
+	}
+	return oauth2.Endpoint{
+		AuthURL:  "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/authorize",
+		TokenURL: "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/token",
+	}
+}

vendor/vendor.json

@@ -879,10 +879,10 @@
 			"revisionTime": "2017-09-28T00:25:42Z"
 		},
 		{
-			"checksumSHA1": "aoeqnmNuZdqxHG0/8bhKNLfRaY4=",
+			"checksumSHA1": "91mzAbqHQ6AAK65DzB4IkLOcvtk=",
 			"path": "golang.org/x/oauth2/microsoft",
-			"revision": "b9780ec78894ab900c062d58ee3076cd9b2a4501",
-			"revisionTime": "2017-02-14T22:24:16Z"
+			"revision": "ec22f46f877b4505e0117eeaab541714644fdd28",
+			"revisionTime": "2018-05-28T20:23:04Z"
 		},
 		{
 			"checksumSHA1": "FUo16zuO20jOcdCUu0IgiF51koE=",