.travis.yml: Update jaruga's email.

I cannot receive Travis's notification via jaruga@ruby-lang.org, while another
committer with their @ruby-lang.org email can receive the notification.
The issue is my ruby-lang.org email address specific.
So far we haven't seen the root cause even with the Travis support's help.

Because of that, I change the email address to my personal email address.

.travis.yml

@@ -148,6 +148,6 @@ notifications:
     on_failure: always
   email:
     recipients:
-      - jaruga@ruby-lang.org
+      - jun.aruga@gmail.com
     on_success: never
     on_failure: always