Update VirtualBox and Vagrant minor versions for September workshop

These are the versions that will be used to build the VM. Looks like
only minor updates.

Pointed the actual download to 2014-09-rc1 now so TAs can test it - this
will be changed later when the final 2014-09 tag is ready. Put the final
filename in the vm_setup page (that should be the only step that needs to
use the filename).

Not much new in the VM except updating Rails to the latest 4.0 version.
Let me know if you want something added/fixed!