Updated default vagrant box to Ubuntu 14.04 from Vagrant Cloud

Creating a new cookbook with berks still produced a Vagrantfile with an Ubuntu 12.04 box. I updated it to the latest released (LTS) version 14.04. 
Since Vagrant Cloud was introduced with the Vagrant 1.5 release, I used the Vagrant Cloud box provided by Chef.
https://vagrantcloud.com/chef/ubuntu-14.04