enable vagrantfile to work with vagrant versions 1 and 2

take code that relies on differences in the api between vagrant
versions and wrap them in configure blocks that will conditionally
execute. this allows backwards compatibility and removes pesky
warnings from Jenkins.
Merge lonnen's patch that quashes the Vagrant warnings on Jenkins. Close #237.
Merge branch 'ui' of github.com:mozilla/dxr into ui to get latest missing changes