Don't highlight documents containing word rainbow by rainbow

Summary: It caused us some troubles because some serious business document was displayed as rainbow to our serious business partners (MFA rainbow.com).

Test Plan: Display document with the word 'rainbow'.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2324