Add mercurial 1.8.4 to GOOD_VERSIONS in tools/onbld/Scm/Version.py
The latest mercurial version supported in 1.8.1. 1.8.4 isn't in any release per se, but I've built it off the oi-build head. I've checked the release [[http://mercurial.selenic.com/wiki/WhatsNew]] and upgrade [[http://mercurial.selenic.com/wiki/UpgradingMercurial]] notes for Mercurial, and these do not indicate any issues in upgrading from 1.8.1 to 1.8.4. I've already made the change locally, and I've been testing 1.8.4 in tandem with changes to the cadmium plugins and have not encountered any issues.