playground: update dependencies
playground: update dependencies

Start using newer versions of dependencies, to reduce tech debt and
make future updates more incremental.

This change was generated by running make update-deps on linux/amd64,
using current versions of all dependencies.

Change-Id: I8e76e010a36cd479aa491f722b4eb8ef9fdf89a0
Reviewed-on: https://go-review.googlesource.com/c/163799
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/playground,golang/playground,golang/playground