.travis.yml: Use Ubuntu 18.04 "Bionic"

This mimicks upstream Hugo's configuration.

See: https://github.com/gohugoio/hugo/blob/master/.travis.yml

.travis.yml

@@ -12,6 +12,6 @@ before_script:
 script:
   - hugo
 sudo: false
-dist: xenial
+dist: bionic
 
 # vim: ts=2 sw=2 et