Make /install/client.html redirect to /using/install-client.html

Because old clients (e.g.: 0.10.1 which is what gets installed if one does `sudo apt-get install tsuru` [sic]; see https://github.com/tsuru/tsuru/issues/923) reference the former page, which is now a [broken link on ReadTheDocs](http://docs.tsuru.io/en/latest/install/client.html).

Fixes: GH-921

```
make doc
```

and now `file:///Users/marca/go/src/github.com/tsuru/tsuru/docs/_build/html/install/client.html` redirects to `file:///Users/marca/go/src/github.com/tsuru/tsuru/docs/_build/html/using/install-client.html`