Add 4-0-dev to allowed Travis base branches

Thanks Benoit for discovering this!

Author: pirj

.travis.yml

@@ -38,3 +38,8 @@ matrix:
     - rvm: ruby-head
     - rvm: rbx-3
   fast_finish: true
+branches:
+  only:
+    - main
+    - /^\d+-\d+-maintenance$/
+    - 4-0-dev