travis: reflect new best-practice travis-ci configuration

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/

.travis.yml

@@ -1,5 +1,6 @@
-sudo: false
 language: node_js
+# Once openjdk/oraclejdk dependencies are resolved, change to dist: xenial
+dist: trusty
 notifications:
   email: false
 node_js: