Update vault lib 1.6.0 to improve performance

[jbsmith7741]

started with PR #767 which added go modules.

steps to replicate go mod update

go get github.com/hashicorp/vault@v1.6.0
go get github.com/coreos/etcd@v3.3.12
go get github.com/ugorji/go@v1.1.1
go get github.com/ugorji/go/codec@none

[jbsmith7741]

@kelseyhightower any chance this could get merged, It makes a big performance difference if you are using vault as a backend.

[abtreece]

@jbsmith7741 I have been intermittently pushing my now divergent fork of this project forward as we use confd at Spreedly. I switched to go modules a couple releases ago and updated the dependencies this weekend.