Include the forgotten :graphviz tag in visualizer_spec.

[sersut]

We have detected this in our Chef DK tests which disable visualizer specs since graphviz is not included with Chef DK. Currently we are running berks specs like this. One of the examples are failing since it requires graphviz but not tagged correctly:

Failures:

  1) Berkshelf::Visualizer#to_png when the graphviz command succeeds builds a png from a Lockfile
     Failure/Error: Unable to find matching line from backtrace
     Berkshelf::GraphvizNotInstalled:
       Berkshelf::GraphvizNotInstalled
     # /opt/chefdk/embedded/apps/berkshelf/lib/berkshelf/visualizer.rb:92:in `to_png'
     # /opt/chefdk/embedded/apps/berkshelf/spec/unit/berkshelf/visualizer_spec.rb:35:in `block (4 levels) in <module:Berkshelf>'