Revert "Install coverage in the virtual environment"
Reverts Vauxoo/docker-odoo-image#201
After https://github.com/Vauxoo/travis2docker/pull/95 I think that the change of coverage venv is not required
Reverts Vauxoo/docker-odoo-image#201
After https://github.com/Vauxoo/travis2docker/pull/95 I think that the change of coverage venv is not required