internal/history,cmd/golangorg: add 1.15 and update base ref

Change-Id: I7b3c89f17b9bcaadaed83272950524d28a8be2bf
Reviewed-on: https://go-review.googlesource.com/c/website/+/247682
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
diff --git a/cmd/golangorg/Makefile b/cmd/golangorg/Makefile
index e1ffcbb..6904ffa 100644
--- a/cmd/golangorg/Makefile
+++ b/cmd/golangorg/Makefile
@@ -4,7 +4,7 @@
 
 .PHONY: usage
 
-GO_REF ?= release-branch.go1.14
+GO_REF ?= release-branch.go1.15
 WEBSITE_HEAD := $(shell git rev-parse HEAD)
 WEBSITE_CLEAN := $(shell (git status --porcelain | grep -q .) && echo dirty || echo clean)
 ifeq ($(WEBSITE_CLEAN),clean)
diff --git a/internal/history/release.go b/internal/history/release.go
index c9da099..c7ead9f 100644
--- a/internal/history/release.go
+++ b/internal/history/release.go
@@ -31,6 +31,10 @@
 // It contains entries for releases of Go 1.9 and newer.
 // Older releases are listed in doc/devel/release.html.
 var Releases = map[GoVer]Release{
+	{1, 15, 0}: {
+		Date: Date{2020, 8, 11},
+	},
+
 	{1, 14, 7}: {
 		Date:     Date{2020, 8, 6},
 		Security: true,