doc + policy: use bionic instead of xenial

[nimbosa]

We need to prefer the newer bionic over xenial as the NEW LTS release of Ubuntu.

Zclassic reference client now uses bionic. Documentation and build policy should be updated accordingly. If bionic cannot be used OR builder insists on using xenial, errors will be encountered and adjustments should be applied on build scripts and/or makefile, which should be documented as well.

Can you reliably reproduce the issue?

If so, please list the steps to reproduce below:

1. Use Ubuntu 16.04.3 LTS (Xenial Xerus) instead of 18.04.1 LTS (Bionic Beaver)
2. Build from master
3. Encounter errors

Expected behaviour

Flawless build, as before.

Actual behaviour + errors

At best, a couple of errors. These will be updated later for proper documentation. (Not on a build machine at the moment. No access ;) )

The version of Zclassic / Zcash you were using:

Run zcashd --version to find out
Zclassic 1.0.14