Use the encrypted endpoint

This updates the Congress API's base URI to use `https://`. We recommend this for all Congress API queries, especially those using location. For more rationale on why, I wrote a recent blog post for Sunlight on it:

https://sunlightfoundation.com/blog/2014/01/28/encrypting-our-congress-api-and-protecting-your-location/

I admit though, I haven't actually tested this commit - I'm assuming net/http will handle the `https://` protocol transparently. If you'd like me to do more testing before merging, I can do that.