Revert "do up force pbuilder update on build"

This reverts commit 6fcd39b06d20f3e561533d0479ae942bf80a6010.

It's true that this will be slower, but the wheezy builds are failing if
we don't do this.

(There is an apt-mirror.front.sepia.ceph.com. Maybe using that would
speed things up?)

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
Merge pull request #6 from kdreyer-inktank/pbuilder-update

Revert "do up force pbuilder update on build"