Fix reversed order of boolean config options.

Summary: See discussion in D4355, this fixes reversed bool logic.

Test Plan:
- Quickly viewed in the web interface to make sure it didn't break anything.
- Saved `ldap.auth-enabled` with correct boolean value in the db.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4357