Use the Congress API's encrypted endpoint

This changes the Congress API's base URL to use our encrypted endpoint (`https://`). We recommend all API users use it, especially for queries involving user location. For more rationale, you can read our recent blog post on it:

http://sunlightfoundation.com/blog/2014/01/28/encrypting-our-congress-api-and-protecting-your-location/

I admit, I have not actually tested this change, I'm assuming Django's HTTP library can handle this transparently. If you'd like me to do more testing pre-merge, I can do that.