Reduce timeout now that Travis supports concurrency limiting

Re. https://github.com/travis-ci/travis-web/pull/289

See: https://travis-ci.org/guardian/scribe/settings

Will follow for plugins.

![image](https://cloud.githubusercontent.com/assets/921609/3624668/1bcb1b4a-0e60-11e4-929b-32021d78627b.png)