Add a repository_short_name value to the template

Hi, 

First PR, starting with an easy one referring to travis-ci/travis-ci#1426 . I don't know if **repository_short_name** sounds good to you. If it doesn't I will change with pleasure.

The best option would be to rename **repository** in **repository_slug** and add a **repository_name** field but it will break the current API.

I will change the doc right after the merge. It's [here](https://github.com/travis-ci/docs-travis-ci-com/blob/gh-pages/user/notifications.md#irc-notification) right ?