Document xvfb in travis config

X virtual frame buffer setup in the `.travis.yml` should be documented.
```yml
before_install:
- sh -e /etc/init.d/xvfb start
```

Review the `.travis.yml` file and its `before_install` xvfb command first. Add documentation explaining the X virtual frame buffer setup and its purpose in the Travis configuration; the issue is done when a contributor can understand why this command is required.

Written by the indexing model from the issue text.