Reliably set permissions on /var/www.

When I tried using ubuntu/trusty (14.04) as the base box, I ran into a problem.  After `vagrant up` I got an error when drush tried to copy the project into /var/www:  `Directory /var/www exists, but is not writable.`

Maybe this is some change based on the update to Ubuntu, but my guess is that the difference is in the base box:  vagrant is already in the admin group in the 12.04 box, but not in 14.04.

I could restart with `vagrant provision` and everything worked, but I prefer if `vagrant up` works cleanly.