Require YAML in Vagrantfile (#344)
Require YAML in Vagrantfile (#344)

On some configurations, `vagrant up` was failing with
  NameError: uninitialized constant YAML
cmclaughlin/cabot,maks-us/cabot,bonniejools/cabot,maks-us/cabot,cmclaughlin/cabot,cmclaughlin/cabot,arachnys/cabot,arachnys/cabot,cmclaughlin/cabot,maks-us/cabot,maks-us/cabot,bonniejools/cabot,arachnys/cabot,bonniejools/cabot,arachnys/cabot,bonniejools/cabot