script URL fixed in README

The correct URL is now on `raw.githubusercontent.com` instead of `raw.github.com`. This commit also adds `-L` to tell `curl` to follow redirections, in case the URL changes again.

See:

```
$ curl -I https://raw.github.com/Netflix-Skunkworks/jenkins-cli/master/jenkins-static
HTTP/1.1 301 Moved Permanently
Date: Wed, 17 Dec 2014 00:13:13 GMT
Server: Apache
Location: https://raw.githubusercontent.com/Netflix-Skunkworks/jenkins-cli/master/jenkins-static
…
```