travis: Upgrade from xenial, default (16.04) -> Bionic (18.04)

tony · tony · commit 8d796d6a2f7e · 2019-12-23T16:08:16.000-06:00
See also: https://docs.travis-ci.com/user/reference/overview/
diff --git a/.travis.yml b/.travis.yml
@@ -1,5 +1,6 @@
 language: python
 sudo: false
+dist: bionic  # 18.04
 python:
   - 2.7
   - 3.6
