More recent Ubuntu dist for Travis CI build

otherwise the following error is thrown:

    The command "npm config set spin false" failed and exited with 1 during .

cf.
https://travis-ci.community/t/the-command-npm-config-set-spin-false-failed-and-exited-with-1-during/12909
nodejs/node#42351 (comment)
https://docs.travis-ci.com/user/reference/overview/

TODO: Comment out "dist: focal" once focal becomes the Travis CI default
Linux distrib and that Node.js 18 runs fine by default.

.travis.yml

@@ -1,3 +1,6 @@
+
+dist: focal
+
 language: node_js
 
 cache: