reduce redirect on download of virtualenv.py

This is pointing to an older GitHub endpoint. Each request will have to
perform a redirect to get the correct file.

```console
$ curl -IL https://raw.github.com/pypa/virtualenv/1.11.X/virtualenv.py
HTTP/1.1 301 Moved Permanently
Date: Mon, 22 Sep 2014 18:09:21 GMT
Server: Apache
Location:
https://raw.githubusercontent.com/pypa/virtualenv/1.11.X/virtualenv.py
Accept-Ranges: bytes
Via: 1.1 varnish
Age: 0
X-Served-By: cache-jfk1023-JFK
X-Cache: MISS
X-Cache-Hits: 0
Vary: Accept-Encoding

HTTP/1.1 200 OK
Date: Mon, 22 Sep 2014 18:09:22 GMT
Server: Apache
Access-Control-Allow-Origin: https://render.githubusercontent.com
Content-Security-Policy: default-src 'none'
X-XSS-Protection: 1; mode=block
X-Frame-Options: deny
X-Content-Type-Options: nosniff
Strict-Transport-Security: max-age=31536000
ETag: "0329fc98c0382822c458c374945509ba5a8d3ffc"
Content-Type: text/plain; charset=utf-8
Cache-Control: max-age=300
Content-Length: 98477
Accept-Ranges: bytes
Via: 1.1 varnish
X-Served-By: cache-jfk1026-JFK
X-Cache: MISS
X-Cache-Hits: 0
Vary: Authorization,Accept-Encoding
Expires: Mon, 22 Sep 2014 18:14:22 GMT
Source-Age: 0
```