PostgreSQL 9.6 fails to install on Ubuntu

Installing only the packages:

sudo apt-get install -y postgresql-9.6 postgresql-client-9.6
postgresql-server-dev-9.6

results in a broken install on Ubuntu version 16.04 (tested on
Travis). A similar packages combination works for all the other
supported and recently unsupported versions, from 9.4 to 13.

After packages are installed the database fails to start with the error:

systemd[1]: Starting LSB: PostgreSQL RDBMS server...
postgresql[5782]: * Starting PostgreSQL 9.6 database server
postgresql[5782]: * Error: cluster_port_ready: could not find psql binary
postgresql[5782]: ...fail!
systemd[1]: postgresql.service: Control process exited, code=exited status=1
systemd[1]: Failed to start LSB: PostgreSQL RDBMS server.
systemd[1]: postgresql.service: Unit entered failed state.
systemd[1]: postgresql.service: Failed with result 'exit-code'.

Complete build log is in [1]; the complete install script is in [2].

[1] https://travis-ci.org/github/reorg/pg_repack/jobs/731695983
[2] https://github.com/reorg/pg_repack/blob/2a4c67e09454732c595995b5b091a671b7bf0e7e/regress/travis_prepare.sh

I've tried installing the generic `postgresql-client` package too but
build fails in other amusing ways (e.g. missing clang 6.0, even if I
doubt PG 9.6 needs it).

This is being a stopper for releasing pg_repack for PG 13, so I will
temporarily remove PG 9.6 from the testing grid.

Thank you

-- Daniele