switch jsdom to jsdom-little to do away with the dependency on contextify

Contextify is what is breaking thresher under node-webkit. I tried to produce a nw-gyp build but there were just too many errors and I'd rather not fork dependencies two levels down from thresher to edit their C bindings...

jsdom-little was created for the very purpose of being jsdom without contextify (which creates issues in a number of other contexts too). With just the hotswap the test suite runs without a hitch.