Better Travis irc bot notifications

`use_notice` will make the bot use notices instead of regular messages,
`skip_join` will disable the bot's joining before sending messages, parting after sending.
`skip_join` will require the removal of the `NO_EXTERNAL_MSGS flag` (n) on the IRC channel.

Detailed info can be found here: http://docs.travis-ci.com/user/notifications/

I didn't open an issue because this is trivial and any discussion can take place here (no harm done if it's rejected).