Only define VAGRANTFILE_API_VERSION if not yet defined

Sometime VAGRANTFILE_API_VERSION is already defined in $HOME/vagrant.d/Vagrantfile.
This will result in the following issue when trying to define VAGRANTFILE_API_VERSION
again in a Vagrantfile of a box.

---snip---
.../Vagrantfile:5: warning: already initialized constant VAGRANTFILE_API_VERSION
$HOME/.vagrant.d/Vagrantfile:1: warning: previous definition of VAGRANTFILE_API_VERSION was here
---snap---