Fix Virtualbox Guest Additions install

#3192 forgot to update the Virtualbox Guest Addtion version on https://github.com/dotcloud/docker/blob/f216448c823d1aa3dd661935d60da390d463039d/Vagrantfile#L73  

this PR attempts to group the version and checksum so further updates to VGA does not hit the same bug. 

/cc @unclejack