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