.travis.yml: Try another way to be notified.

I <jaruga@ruby-lang.org> haven't received the notifications since August 2022.
So, try another way to be notified written on the document below.

https://docs.travis-ci.com/user/notifications/#configuring-email-notifications

.travis.yml

@@ -146,4 +146,7 @@ notifications:
     on_success: never
     on_failure: always
   email:
-    - jaruga@ruby-lang.org
+    recipients:
+      - jaruga@ruby-lang.org
+    on_success: never
+    on_failure: always