Use define() instead of PHP 5.3-only global 'const' in upgrade_schema.php

Summary:
This global 'const' syntax was introduced in PHP 5.3:

http://www.php.net/manual/en/language.constants.syntax.php

We're PHP 5.2.x elsewhere so just use define(). Made the constant a little more
specific too.

Test Plan:
Ran upgrade_schema.php script.

Reviewed By: Girish
Reviewers: tuomaspelkonen, Girish, davidrecordon
CC: jungejason, aran, epriestley, Girish
Differential Revision: 190