Add a deprecation notice to the index.

This site is currently misleading because it claims to hold the latest
version, but actually doesn't since the download site was moved.

Addresses https://github.com/mitchellh/vagrant/issues/4773

Screenshot:
![screen shot 2014-11-21 at 11 28 38](https://cloud.githubusercontent.com/assets/5560/5141779/94d352f4-7171-11e4-8387-626e60868b91.png)