Allow Travis CI's IRC bot to post without joining (graphql#220)

See https://docs.travis-ci.com/user/notifications#IRC-notification for more information.

Note that this requires removing the `n` flag on the IRC channel.
See https://freenode.net/kb/answer/channelmodes

> n (prevent external send): Users outside the channel may not send messages to it. Keep in mind that bans and quiets will not apply to external users.

.travis.yml

@@ -16,6 +16,7 @@ notifications:
     use_notice: true
     on_success: change
     on_failure: change
+    skip_join: true
     channels:
       - chat.freenode.net#graphql