Use the correct Content-Type for the Aphlict status route

Summary: The /status/ route for the Aphlict server returns JSON.

Test Plan:

> curl -I http://127.0.0.1:22281/status/
HTTP/1.1 200 OK
Content-Type: application/json
Date: Fri, 02 Jan 2015 13:08:34 GMT
Connection: keep-alive

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11144