Updated clustering example

There are two things done here:
 * Use a NumPy file instead of a pickle for loading the clustering example's data. It's more secure, generally, and a bit smaller, too. I uploaded the new file to the examples repo, so could someone update the web server? (Travis will fail because of this.)
 * Use `scipy.cluster` and `scipy.spatial` instead of `hcluster`. It seems like all `hcluster` does was folded into SciPy, so there's no need for it. Plus, it's old and doesn't support Python 3 anyway.