Add missing note about `Vagrant::Environment::VAGRANT_HOME` deprecation to the changelog

[This commit](https://github.com/mitchellh/vagrant/commit/fbdd46a130890f03eada3bd607d61ef7a4b75671) removed the constant and since I've seen quite a few people relying on it for things like [apt packages cache](https://gist.github.com/millisami/3798773) I think it is worth mentioning on the changelog ;-)