Fix travis.yml (trusty > xenial)

Builds otherwise fail without this.

Ref: <https://travis-ci.community/t/travis-is-downloading-older-chrome-versions-in-place-of-chrome-stable/5040/4>

Author: niftylettuce

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 sudo: required
-dist: trusty
+dist: xenial
 node_js: stable
 addons:
   sauce_connect: true