Ubuntu Xenial, which is now being rolled out as the default Linux distro
for Travis builds following the end of life of Ubuntu Trusty, does not
start services such as MySQL or PostgreSQL unless told to. Unfortunately
simply adding both to the list of services causes problems with
PostgreSQL tests, even under Trusty and if we explicitly request
PostgreSQL-9.6. See e.g.

https://api.travis-ci.org/v3/job/538735642/log.txt

Make sure we stick with Trusty until after the above has been resolved.