don't check for berks executable and correct version if the plugin is disabled

Don't try to check for the berks executable and correct version if the plugin is disabled.

It's related to [this comment](https://github.com/test-kitchen/test-kitchen/issues/575#issuecomment-68399469) where we discussed to disable vagrant-berkshelf in test-kitchen/kitchen-vagrant (because test-kitchen itself already resolves the dependencies via berks).