Use https by default for all API requests

Flickr API Going SSL-Only on June 27th, 2014

See:
http://code.flickr.net/2014/04/30/flickr-api-going-ssl-only-on-june-27th-2014/

Important details:

* Non SSL access is being removed by Flickr, all new requests must go
  to the https://api.flickr.com endpoint

* All requests to http://www.flickr.com will be redirected to the https
  version.
Merge pull request #20 from brk3/feature/ssl-only

Use https by default for all API requests