api: add go1.5.txt API file, reset next.txt

Fixes #11752

Change-Id: If4196b4bbba4f1485608bfafa98fa54781177273
Reviewed-on: https://go-review.googlesource.com/12325Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@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"),
+		"-c", file("go1", "go1.1", "go1.2", "go1.3", "go1.4", "go1.5"),
 		"-next", file("next"),
 		"-except", file("except")).CombinedOutput()
 	if err != nil {