fix downloads from gh-pages sites

[rvagg]

Underscore, Backbone, Modernizr, MinifiedJS all seem to suffer intermittently from this problem; build gets "zero-length source", as does a curl to the source url (but curl again and you get it!).

[rvagg]

$ curl -vv http://underscorejs.org/underscore.js
* Adding handle: conn: 0x110bdf0
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0x110bdf0) send_pipe: 1, recv_pipe: 0
* About to connect() to underscorejs.org port 80 (#0)
*   Trying 207.97.227.245...
* Connected to underscorejs.org (207.97.227.245) port 80 (#0)
> GET /underscore.js HTTP/1.1
> User-Agent: curl/7.32.0
> Host: underscorejs.org
> Accept: */*
> 
< HTTP/1.1 302 Found
< Connection: close
< Pragma: no-cache
< cache-control: no-cache
< Location: /underscore.js
< 
* Closing connection 0