Fix `berks viz` when `pwd` contains spaces

On my OS X laptop, I have spaces in my `pwd` where I do cookbook work.  I tried doing `graph viz`, but it complained with the following:

```
$ berks viz
The Graphviz command `dot -T png /var/folders/x7/8sms15l54t11h6j6nlvgk8hh0000gn/T/dotdotfile20140828-72973-1wniqtf -o /Users/ameir/Aptana Studio 3 Workspace/Chef/cookbooks/my_cookbook/graph.png` failed to execute properly. Here is the standard error from the command:

Error: dot: can't open Studio
Error: dot: can't open 3
Error: dot: can't open Workspace/Chef/cookbooks/my_cookbook/graph.png
```

This PR simply adds quotes to fix this.