Switch back go upstream bwmarrin/discordgo

Commit https://github.com/bwmarrin/discordgo/commit/ffa9956c9b41e8e2a10c26a254389854e016b006 got merged in.

vendor/golang.org/x/crypto/poly1305/sum_noasm.go

@@ -0,0 +1,14 @@
+// Copyright 2018 The Go 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 s390x,!go1.11 !arm,!amd64,!s390x gccgo appengine nacl
+
+package poly1305
+
+// Sum generates an authenticator for msg using a one-time key and puts the
+// 16-byte result into out. Authenticating two different messages with the same
+// key allows an attacker to forge messages at will.
+func Sum(out *[TagSize]byte, msg []byte, key *[32]byte) {
+	sumGeneric(out, msg, key)
+}