Update twitter reference to be secure.

The insecure link is currently just redirected to the secure link by twitter. This update goes directly to the secure version that people will end up at anyway.

Below is a capture of the insecure version being redirected to the secure version by Twitter:

    $ HEAD -sSe 'http://twitter.com/intent/tweet'
    HEAD http://twitter.com/intent/tweet
    301 Moved Permanently
    Date: Thu, 26 Dec 2013 20:48:23 UTC
    Location: https://twitter.com/intent/tweet
    Server: tfe
    Content-Length: 0
    Client-Date: Thu, 26 Dec 2013 20:48:22 GMT
    Client-Peer: 199.59.149.198:80
    Client-Response-Num: 1
    Set-Cookie: guest_id=v1%3A138809090301612382; Domain=.twitter.com; Path=/; Expires=Sat, 26-Dec-2015 20:48:23 UTC
    
    HEAD https://twitter.com/intent/tweet
    200 OK