Make pbr a build-time only dependency

This lets you build swift packages that don't require pbr
to be installed at all. You would need pbr on the machine running
rpmbuild / debuild, but not on the machines that install the packages.

Unfortunately, this does not make swift able to be
installed via pip 0.3.1 on Lucid; you'll need to uninstall the system
python-pip package and install a new pip some other way. Given that
pip < 1.3 doesn't perform SSL certificate validation for pypi (trivial
MITM attack, anyone?), you'd probably want to get a new pip anyway.

Change-Id: Ia50a229c5ae4dd2158beeaa953619b5e8f987c55