Travis failures with Postgres tests

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Description

Travis builds are failing semi randomly now that we're testing against Postgres. Tests pass when rerun manually. I suspect a database contention or cleaning issue, but I haven't reproduced it reliably yet.

Comienza examinando la configuración de Travis y la configuración de pruebas de Postgres; después, vuelve a ejecutar la suite que falla para comparar los fallos intermitentes con las ejecuciones manuales. Investiga la contención de la base de datos o el comportamiento de limpieza reportados; se considera terminado cuando las pruebas de Postgres pasan de forma consistente en Travis.