fix vagrant api version

"Vagrant failed to initialize at a very early stage:

There was an error loading a Vagrantfile. The file being loaded
and the error message are shown below. This is usually caused by
a syntax error.

Path: /opt/vagrant-phalcon/Vagrantfile
Message: undefined method `config' for Vagrant:Module"

related to: https://github.com/puphpet/puphpet/issues/1