Find Vagrantfile with ascending directory.

## Vagrantfile lookup path
> Vagrant climbs up the directory tree looking for the first Vagrantfile it can find, starting first in the current directory.

http://docs.vagrantup.com/v2/vagrantfile/index.html

## Improvement
`serverspec-init` climbs up the directory tree looking for the first `Vagrantfile` it can find, starting first in the current directory.

## Log
```
➜  grandchild  pwd
/tmp/serverspec/child/grandchild
➜  grandchild  ls /tmp{,/serverspec,/serverspec/child,/serverspec/child/grandchild}/Vagrantfile
ls: /tmp/Vagrantfile: No such file or directory
ls: /tmp/serverspec/Vagrantfile: No such file or directory
ls: /tmp/serverspec/child/Vagrantfile: No such file or directory
ls: /tmp/serverspec/child/grandchild/Vagrantfile: No such file or directory
➜  grandchild  ./bin/serverspec-init
Select a backend type:

  1) SSH
  2) Exec (local)

Select number: 1

Vagrant instance y/n: y
Auto-configure Vagrant from Vagrantfile? y/n: y
Vagrantfile not found in directory!
➜  grandchild  cd ../..
➜  serverspec  vagrant init precise64
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.
➜  serverspec  cd child/grandchild
➜  grandchild  ./bin/serverspec-init
Select a backend type:

  1) SSH
  2) Exec (local)

Select number: 1

Vagrant instance y/n: y
Auto-configure Vagrant from Vagrantfile? y/n: y
 + spec/
 + spec/default/
 + spec/default/httpd_spec.rb
 + spec/spec_helper.rb
 + Rakefile
```