ci(travis): use `dist: bionic`

* Automated using https://github.com/myii/ssf-formula/pull/22

.travis.yml

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # vim: ft=yaml
 ---
+dist: bionic
 stages:
   - test
   - lint
@@ -10,7 +11,6 @@ stages:
 sudo: required
 cache: bundler
 language: ruby
-dist: xenial
 
 services:
   - docker