I had the same assumption as @davehunt.

Running a brief check, it looks like it. Removing the war file and making the download url invalid gives the following traceback:

>>Downloading Jenkins 1.509.4 from http://mirrors.jenkins-ci.org/war-stable/1.509.4/jenkins.wa
Download failed.
Traceback (most recent call last):
  File "start.py", line 42, in <module>
    download_jenkins()
  File "start.py", line 30, in download_jenkins
    r = urllib2.urlopen(JENKINS_URL)
  File "/usr/lib/python2.7/urllib2.py", line 126, in urlopen
    return _opener.open(url, data, timeout)
  File "/usr/lib/python2.7/urllib2.py", line 406, in open
    response = meth(req, response)
  File "/usr/lib/python2.7/urllib2.py", line 519, in http_response
    'http', request, response, code, msg, hdrs)
  File "/usr/lib/python2.7/urllib2.py", line 444, in error
    return self._call_chain(*args)
  File "/usr/lib/python2.7/urllib2.py", line 378, in _call_chain
    result = func(*args)
  File "/usr/lib/python2.7/urllib2.py", line 527, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)
urllib2.HTTPError: HTTP Error 404: Not Found

looks alright to me...