Support for `require_chef_omnibus`

Hi there,

I added the missing support for `require_chef_omnibus` described in https://github.com/opscode/kitchen-vagrant/issues/33

As my colleague Alexander said, this should have been fixed in #4 already, but the current sources don't contain this parameter. Since the implementation has changed a bit, I added my changes to `vagrantfile_creator.rb` using the same pattern for writing into the file as all other methods do by calling `omnibus_block` from the `render` method, just in case the parameter `require_chef_omnibus` is defined.

Best Regards,
Andreas