[Cryptech Tech] Binary packages for Debian Stretch and Ubuntu Bionic

[Rob Austein]

Over the weekend I updated the build system to create binary packages
for Debian Stretch (9) and Ubuntu Bionic (18.04) in addition to the
previous Debian Jessie (8), Ubuntu Xenial (16.04), and MacOS Homebrew.

The Stretch and Bionic packages have not yet been tested beyond the
pbuilder package builds running to completion.  Since our client side
code isn't doing anything very exotic and the OS-specific comms voodoo
is in third-party Python libraries which we were backporting in the
first place, the same old code should Just Work on the new platforms,
but a volunteer who has time to confirm this would be welcome.

Updated installation instructions in the same place as always:

https://wiki.cryptech.is/wiki/BinaryPackages