Blacklist Vagrantfile like Rakefile in FileName cop

There may come a time when we fix an error like `Use snake_case for source file names` in default Vagrantfile .


It is as follows:

```bash
$ mkdir ~/work/vagrant-issue && cd ~/work/vagrant-issue

$ vagrant -v
Vagrant 1.6.1

$ vagrant init
A `Vagrantfile` has been placed in this directory. You are now
ready to `vagrant up` your first virtual environment! Please read
the comments in the Vagrantfile as well as documentation on
`vagrantup.com` for more information on using Vagrant.

$ rubocop -v
0.21.0

$ rubocop Vagrantfile
Inspecting 1 file
C

Offenses:

Vagrantfile:2:1: C: Use snake_case for source file names.
# vi: set ft=ruby :
^
:
```