Change default fonts to Liberation form badly-licensed Microsoft fonts.

Here is what I did:

* Build master
* Run `./servo http://exyr.org`. Renders fine.
* Uninstall the `ttf-ms-fonts` package.
* Run `./servo http://exyr.org`. Fails with `No matching font(s), are the appropriate fonts installed?`
* Apply the changes in this PR
* Run `./servo http://exyr.org`. Renders fine.