bugfix: this depends on recipe[ohai] having created directories

Hi there.

In the default recipe, you [depend on the ohai plugin path](https://github.com/Afterglow/chef-ipmi/blob/d462156528a6eadd7e528619e84175e371da136c/recipes/default.rb#L39) to be created.  Since the attribute used actually comes from the ohai cookbook, I suppose this should be included (and, well, it works this way).

Thanks for the cookbook, it's really handy!
Cheers
Stephan