Use Ubuntu 20.04 at Travis CI

yahonda · yahonda · commit 088fc2510231 · 2023-05-21T15:04:22.000+09:00
* The Ubuntu 20.04 (Focal Fossa) Build Environment https://docs.travis-ci.com/user/reference/focal/
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,4 @@
-dist: xenial
+dist: focal
 sudo: required
 
 jdk:
