Remove depency on metrics-web's foreignkey branch
The foreignkey branch of metrics-web introduced a few database schema changes to establish a foreign key relationship between statusentry and descriptor. This was necessary in order to please Django's database mapper. However, the latest TorStatus version uses its own tables which don't require the foreignkey changes anymore. Once we're really sure we don't need the foreignkey branch anymore (Jeremy, Vlad, Diego?), we should remove the parts in the README saying we need it.