change directory if directory exists

When using Vagrant with Test-Kitchen, Vagrantfile and the directory which this file is stored will be removed after the execution of Test-Kitchen.
In that situation, vagrant-global-status command shows the error like below...
```
/home/ryuzee/cookbooks/dummy/.kitchen/kitchen-vagrant/default-CentOS-64
sh: 1: cd: can't cd to /home/ryuzee/cookbooks/dummy/.kitchen/kitchen-vagrant/default-CentOS-64

/home/ryuzee/tmp/lucid32_test
  default         poweroff (virtualbox)
```

I think it is necessary to check the existence of the directory...
Thanks!