api: promote next.txt to go1.8.txt, update go tool

Change-Id: I37f2893ce14009efe095aac3bc811d650c66bf2a
Reviewed-on: https://go-review.googlesource.com/33761Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/api/run.go

@@ -26,7 +26,7 @@ func main() {
 	}
 
 	out, err := exec.Command("go", "tool", "api",
-		"-c", file("go1", "go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7"),
+		"-c", file("go1", "go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7", "go1.8"),
 		"-next", file("next"),
 		"-except", file("except")).CombinedOutput()
 	if err != nil {