Deprecate #local_deps_path

The config method `#local_deps_path=` is a convenient and short way to share a folder of Babushka deps with the virtual machine. However, it doesn't really do much (it's basically just a convenience method), and it's difficult to customise (it doesn't support NFS, sharing multiple directories, or changing the ID of the shared folder). Somebody unfamiliar with this plugin who reads this in use would have to check the documentation or source code for this plugin, to see that it's just sharing a folder with the virtual machine.

This change outputs a warning to Vagrant's log if `#local_deps_path` is used, explaining that the method is deprecated, and offering an equivalent configuration line that can be used to have the exact same effect as the old method did. It also allows the method to continue working exactly as it has. The purpose is to notify anybody using this method that it's deprecated, and explain the benefits of sharing the folder manually.