revert to upstream url to install hiera-etcd

https://github.com/jumanjihouse/docker-puppet/pull/10 switched
the url to my fork of hiera-etcd.

https://github.com/garethr/hiera-etcd/pull/6 has been merged, so
it's better imho to use the upstream url to ensure we always have
the latest version of hiera-etcd.