Get rid of file_get_contents($uri)

Summary: It requires allow_url_fopen which we don't check in setup and our installation is about to disable it.

Test Plan:
Login with OAuth.
/oauth/facebook/diagnose/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2787