Fix raw path for kerl download

The `curl` command kept silently failing, finally ran with `-i`:

    $ curl -i https://raw.github.com/spawngrid/kerl/master/kerl
    HTTP/1.1 301 Moved Permanently
    Date: Sun, 27 Apr 2014 01:21:14 GMT
    Server: Apache
    Location: https://raw.githubusercontent.com/spawngrid/kerl/master/kerl

Hostname updated in README (could alternatively or additionally add `-L` as an argument to `curl`)