when crushing a site, we should delete the files when they exist

If we try to delete the files after, this happens:

````
Executing: rm -rf /var/www/usf
Executing: rm -rf /home/vagrant/usf/sites/default/files

and with this change:

````
Executing: rm -rf /var/www/usf/sites/default/files
Executing: rm -rf /var/www/usf