Add csp_reporter to caching proxy

Summary: Whitelist csp_reporter path for cashing proxy so browser can send reports to it

Test Plan: - Get logs from csp_reporter (already put this live and restarted, it does work)

Reviewers: alpert

Reviewed By: alpert

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D10757