build: stop testing release-branch.go1.5

Normal Go policy is to support the past two released Go version, which
is currently Go 1.6 and Go 1.7.

Change-Id: I8db9495582d40e82ceb4ccf52ea22ec3969f6ae5
Reviewed-on: https://go-review.googlesource.com/31767
Reviewed-by: Quentin Smith <quentin@golang.org>
diff --git a/app/build/dash.go b/app/build/dash.go
index 2a32eeb..7d84613 100644
--- a/app/build/dash.go
+++ b/app/build/dash.go
@@ -189,4 +189,5 @@
 // from testing sub-repos against these branches.
 var hiddenBranches = map[string]bool{
 	"release-branch.go1.4": true,
+	"release-branch.go1.5": true,
 }