Force using system site packages in the virtualenv

Workaroung for sippable.com lack of support for the
--system-site-packages, as per
https://github.com/Shippable/support/issues/241
Add "-y" (--yes) option to apt-get commands

To allow running the script unattended in systems without that
default.
Merge pull request #140 from dreispt/workaround-shippable-virtualenvs

Force using system site packages in the virtualenv