[FIX] Packaging: Debian Dockerfile for nightly tests

Remove installation of v8 requirements in package.dfdebian as this
is unnecessary for v7. This fixes a bug occuring during the installation
of openerp (precisely when installing reportlab dependencies), because
reportlab now requires a more recent version of pillow than 2.5.1.
Also use pip instead of /usr/local/bin/pip in test_tgz (package.py)
as pip script isn't installed there anymore.