Invalid endpoint returns 403 instead of 404

[njgingrich]

Seems related to #228.

Was testing out some endpoints, and the endpoint https://api.propublica.org/congress/v1/116/senate/floor_updates/2017/05/02.json returns a 403, with the following response:

{
	"message": "Missing Authentication Token"
}

The correct endpoint is https://api.propublica.org/congress/v1/senate/floor_updates/2017/05/02.json, without the current congress parameter (116).

It would make more sense in my mind for that invalid endpoint to return a 404, since it made me double check I was in fact sending the correct header, though it's not really that impactful of an issue.