readme: point travis-worker to github repo

$ curl -i about.travis-ci.org/docs/dev/worker/
HTTP/1.1 404 Not Found
Server: GitHub.com
Content-Type: text/html
Content-Length: 9371
Accept-Ranges: bytes
Date: Mon, 09 Sep 2013 15:18:12 GMT
Via: 1.1 varnish
Age: 0
Connection: keep-alive
X-Served-By: cache-sv63-SJC3
X-Cache: MISS
X-Cache-Hits: 0
X-Timer: S1378739892.310816526,VS0,VE74
Vary: Accept-Encoding
[snip]