Don't upgrade setuptools or distribute

It turns out that upgrading setuptools and distribute in the venv
we use to build the mirror causes MySQL-python to error out. However,
as of virtualenv 1.10.1, there's a new enough version of pip (1.4)
so it's actually not necessary for us to explicitly upgrade.

Change-Id: Ib08064ad1ce674323cd5dec87b7a9911c723d12a