.travis.xml: require xenial distribution

mrjbq7 · mrjbq7 · commit 115bff9ff722 · 2019-04-19T14:12:48.000-07:00
diff --git a/.travis.yml b/.travis.yml
@@ -6,8 +6,8 @@ python:
   - "3.5"
   - "3.6"
   - "3.7"
-os:
-  - linux
+os: linux
+dist: xenial
 env:
   global:
     - DEPS_DIR=$HOME/dependencies
