add handling both vagrant 1.5 and 1.6

Hey,
Since Vagrant 1.6, there's no more need for vagrant-windows plugin.
Currently if you try using opentable images in 1.6, you'll get errors about missing plugin.
I've created a config that should support both old 1.5 versions and newer 1.6 versions.
Obviously it needs to be applied on all of the configs, but first I need to know if you like the idea at all.