Install ubuntu-system-service for Travis testing

This package provides the org.freedesktop.hostname1 service that is used
to test udiskie.common.
Install problematic dependencies in before_install

Use the virtualenv --system-site-packages option to inherit these
hard-to-build dependencies into the virtual python environment.

Rationale:
PyGObject and dbus-python turn out to be pretty hard to install using
the virtualenv environment, especially on python3 where the configure
script fails to find the python headers.