Fix broken link in Vagrantfile

I tried setting up a Pombola vagrant box today. After I'd copied the example Vagrantfile into the same directory as the `pombola` directory, `vagrant up` failed with:

```
    default: Running: inline script
==> default: stdin: is not a tty
==> default: sh: 0: Can't open install-site.sh
```

The reason `sh` couldn't open `install-site.sh` is because…

```
curl -s -O https://raw.github.com/mysociety/commonlib/dev-argument/bin/install-site.sh
```

…didn't output anything, because `curl` encountered a redirect (to raw.githubusercontent.com) and didn't follow it. No `install-site.sh` file was created.

This pull request fixes that – first by calling the correct URL in the first place, and second by following redirects in case GitHub moves the file again.