cmd/release: use Mac OS 10.11 builders for Mac releases

Change-Id: I0d82e62f5d294abcbc5225c9eb08aaf9422e6e1e
Reviewed-on: https://go-review.googlesource.com/28710
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/release/release.go b/cmd/release/release.go
index c91d4da..58708b0 100644
--- a/cmd/release/release.go
+++ b/cmd/release/release.go
@@ -203,7 +203,7 @@
 		OS:      "darwin",
 		Arch:    "amd64",
 		Race:    true,
-		Builder: "darwin-amd64-10_10",
+		Builder: "darwin-amd64-10_11",
 	},
 	{
 		OS:       "linux",