Switch to the Congress API's HTTPS endpoint

Hi, I'm the maintainer of Sunlight's Congress API. In January, we started strongly recommending that all API traffic [use our HTTPS endpoint by default](http://sunlightfoundation.com/blog/2014/01/28/encrypting-our-congress-api-and-protecting-your-location/).

This pull request changes all references to the `http://` endpoint to the `https://` endpoint. I'm extremely rusty in PHP, so I have not actually tested this! I'm assuming the network calls you're making in PHP handle a protocol change transparently. I'm happy to help further if I can.