imports: update zstdlib to Go 1.9

We're at beta 2 already, so the API is likely stable.
If anything gets rolled back, we can update imports again.

Change-Id: I87e0adde062e45cab16e2bee2f929c1ac1bb828d
Reviewed-on: https://go-review.googlesource.com/49150
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed