Remove RailsInstaller and duplicated VM instructions, move vm_setup under /installfest

This will fix #142 (didn't get any objections there).

I think we're all in favor of trying to get everyone set up with the VM. Keeping non-VM instruction in the OS X 10.5 page. Remove duplication of the VM instructions in the regular (i.e. 10.6-10.9) OS X page.

This gets the vm_setup page a bit closer to being the same for Windows and Mac except for the installers, which I think is a good goal.