Remove explicit email notifications from .travis.yml

Explicit email addresses don't play well with forks, i.e. the given
mail receivers are *always* notified instead of the author and the
committer. This led to build notifications sent to me instead of Matt
when he committed something in his own fork.

The default rules are much better in this case. The drawback is that
*only* the committer and author get notified and there is obviously
no way to add anybody that gets notified *additionally* to author
and committer.

https://docs.travis-ci.com/user/notifications/#how-is-the-build-email-receiver-determined

.travis.yml

@@ -67,8 +67,12 @@ after_success:
       cd -
     fi
 
-notifications:
-  email:
-    recipients:
-      - fabien@onepost.net
-      - albrechts.fltk@online.de
+# explicit email notifications don't play well with forks: don't use them
+# default: committer and author get notifications
+# we're now using defaults for everything, hence settings are commented
+# out, left only for documentation purposes...
+
+#notifications:
+#  email:
+#    on_success: change # default: change, may be: never
+#    on_failure: always # default: always