Go get multihash fails compilation due to sha3.NewLegacyKeccak512

[jburnett]

Following readme
$ go get github.com/multiformats/go-multihash

Results
# github.com/multiformats/go-multihash go/src/github.com/multiformats/go-multihash/sum.go:136:7: undefined: sha3.NewLegacyKeccak512

$ go version go version go1.13.8 linux/amd64

on Ubuntu 20.04 LTS