Replace leading double underscore in function names by single underscore

Summary:

PHP reserves all symbols starting with __ as magical. http://php.net/userlandnaming.rules

I didn't touch third-party S3 library.

Test Plan: /

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2077