runtime: update heap dumper header to 1.6.

Change-Id: Ic2a326d41783fb591148748dbcccfd3855091437
Reviewed-on: https://go-review.googlesource.com/18912Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/heapdump.go

@@ -639,7 +639,7 @@ func dumpmemprof() {
 	}
 }
 
-var dumphdr = []byte("go1.5 heap dump\n")
+var dumphdr = []byte("go1.6 heap dump\n")
 
 func mdump() {
 	// make sure we're done sweeping