You can build your own WAR file from the latest source. Alternatively you can use one of the NightlyBuilds.
(The unit tests now require Java 1.6 to run fully - this was unintentional, probably fixable by adding another jar. There doesn't appear to be an XPath API implementation in 1.5 or something - on my todo list. We now skip that test if necessary with stderr warning. -- MattHillsdon)
On Ubuntu and similar that'd be:
sudo apt-get install sun-java5-jdk ant subversion
Then to build:
svn co http://svn.reviki.org/svn/reviki/trunk reviki cd reviki ant war
reviki.war is the newly created WAR file as referenced on SetupGuide.
Do ask for help if you have any trouble - see feedback on FrontPage.