Trouble finding a working revision
Fragment of a discussion from Extension talk:LiquidThreads
Try using MW1.17 with Liquid threads from the MW1.17 branch or tag. http://svn.wikimedia.org/svnroot/mediawiki/tags/REL1_17_0/extensions/LiquidThreads/ The head of LT failes with the error you give when used with older MW (I think RequestContext is new in MW1.18)