Travis CI fails randomly at running tests

[oyvindhagberg]

Sometimes during the setup phase of Travis CI jobs, it fails to install Postgres 11, and the system ends up with a Postgres 9 server. Subsequently, unit tests fail because Django uses SQL features that aren't supported by Postgres 9. (One example is ON CONFLICT)

Exactly why this happens is yet to be determined.
From https://travis-ci.org/unioslo/mreg/jobs/646850146#L317:

-- Subject: Unit postgresql.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit postgresql.service has begun starting up.
Feb 06 12:50:48 travis-job-12369335-cd59-491b-a751-ad78386366fd postgresql[6046]: PostgreSQL 9.4
Feb 06 12:50:48 travis-job-12369335-cd59-491b-a751-ad78386366fd postgresql[6046]: 11 is not installed!
Feb 06 12:50:48 travis-job-12369335-cd59-491b-a751-ad78386366fd systemd[1]: postgresql.service: Control process exited, code=exited status=31
Feb 06 12:50:48 travis-job-12369335-cd59-491b-a751-ad78386366fd systemd[1]: Failed to start LSB: PostgreSQL RDBMS server.
-- Subject: Unit postgresql.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit postgresql.service has failed.