Try to fix the ticket #1147

I filed a ticket about "pysvn.Client.update function is not available in VCS.py" at http://trac.mcs.anl.gov/projects/bcfg2/ticket/1147
This is tested only on my RHEL6 machine.

I am not sure if this would work for all the other distros but as long as pysvn behaves the same way, I think it would work.