2017/state-of-go/stdlib/sort: fix build for Go 1.7 and below

Change-Id: Ic57fac2538bba21501e9a2503a60804b9161d2cc
Reviewed-on: https://go-review.googlesource.com/37387
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>