8.0.2.0-rc6: Install libjpeg8-dev since Odoo needs it in case of demo data

No need to install libjpeg-turbo8 since it's a dependency of
libjpeg8-dev, so it will be installed automatically.

Error without the lib:
Database creation error: "decoder jpeg not available" while parsing /opt/odoo/sources/odoo/openerp/addons/base/base_demo.xml:38

Source:
https://stackoverflow.com/a/10109941/4657755