Update from Ubuntu 16.04 to 18.04

.travis.yml

@@ -7,7 +7,7 @@
 # python: one day so making the migration simpler). "shell" and
 # "minimal" are aliases.
 language: shell
-dist: xenial
+dist: bionic
 group: travis_latest
 os:
   - linux

docs/changes/1621.feature

 Update bundled libuv from 1.36.0 to 1.38.0.
+
+Travis CI now uses Ubuntu 18.04, an upgrade from Ubuntu 16.04.