[zapps-wg] Powersoftau deterministic build

[Devrandom]

I have submitted a pull request for doing a deterministic build of the
Golang powersoftau implementation.  The pull request is here:
https://github.com/FiloSottile/powersoftau/pull/4 . The guest OS is Debian
9 (stretch), due to the requirement for Golang 1.7+.

This was tested as follows:

- Ubuntu 17.10 host with Virtualbox
- Ubuntu 16.04 host with LXC

Other combinations may work, but KVM is known to not work with a Debian
guest.

Until this is merged, you can follow the README but clone my branch instead:

git clone git at github.devrandom/golang-powersoftau

Please try to replicate my resulting SHA256 hash:

c28894877c8948960eaefa0d8f35da6c911031980672b983f361711a7dcb1ec8  taucompute