updated vagrant download link

hey seth :) hope you don't mind me playing around here. like I said, I'm pretty thorough ;)

this page: http://downloads.vagrantup.com/ says that it's actually an archive of old vagrant versions, so I updated the download link to the new one: http://www.vagrantup.com/downloads.html

and also deleted the line "You’ll see a list of version numbers on that first page – just click the very top one to get the latest release of vagrant." since it's no longer relevant :)