travis: switch to ubuntu focal, which is shipped with ruby 2.7 rather than 3.0 on jammy. the github releases dpl provided is incompatible with ruby 3 (travis-ci/dpl#1234)

harababurel · harababurel · commit abdfbfc34414 · 2023-07-24T12:23:01.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,4 @@
-dist: jammy
+dist: focal
 language: rust
 sudo: required
 rust:
