puts a hex.pm package url after published

`mix hex.docs` prints the following after published docs.

```
Hosted at http://hexdocs.pm/ltsv/0.1.0/
```

I like this kindness feature :)
So i bring this feature for `mix hex.publish`.
Maybe we should just make it one line?

    Mix.shell.info("Published at #{Hex.Util.hex_package_url(meta[:name], meta[:version])}")