Travis-CI has or will shortly make in early December 2018 a number of beneficial
changes to their Linux continuous integration testing infrastructure.

Changes that impact pelias/schema are:
* Linux infrastructure combined into one (virtualized), from two previously
  (virtualized and container-based). [0][1]
* Offering a more modern, supported Ubuntu Xenial (16.04 LTS). [2]
* Modest speed improvements from the fully virtualized-based infrastructure.

NOTE: Until openjdk/oraclejdk dependencies can be resolved on modern Ubuntu and
Travis-CI environment, keep the image at Ubuntu Trusty (14.04 LTS).

Projects using "sudo: false" (container-based infrastructure), have been
recommended to remove that configuration soon. In any case, the transition
will happen regardless for projects by December 7, 2018.

[0] https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures
[1] https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
[2] https://docs.travis-ci.com/user/reference/xenial/

A hard switch-over is coming soon.

Due to a limit to header sizes in the latest security releases of
Node.js, combined with Elasticsearch's default of sending lots of
deprecation warning errors as headers, we need to use slightly older
versions of Node.js until either Elasticsearch offers more configuration
options, or Node.js releases a CLI option for the header limit.

See #337 for details