Where a catimages dependency exists in the Ubuntu APT repository for precise, we want them to be provided by the Travis CI build box without needing to build them from source.
This especially applies to non-Python packages, but it also applies to Python packages (see 2.7_with_system_site_packages in .travis.yml).
The process for adding packages to the whitelist is explained at:
https://github.com/travis-ci/apt-package-whitelist
(the same will need to be done for trusty, in the future)