Use virtualenv instruction for travis

From the Travis docs:

    CI Environment uses separate virtualenv instances for each Python
    version. System Python is not used and should not be relied on. If
    you need to install Python packages, do it via pip and not apt.

    If you decide to use apt anyway, note that Python system packages
    only include Python 2.7 libraries on Ubuntu 12.04 LTS. This means
    that the packages installed from the repositories are not available
    in other virtualenvs even if you use the --system-site-packages
    option.

    http://about.travis-ci.org/docs/user/languages/python/#Travis-CI-Uses-Isolated-virtualenvs

See also:

http://stackoverflow.com/questions/13365697/install-python-dbus-in-virtualenv