Pin working jsdom dependency.

It turns out that a change[0] between jsdom 0.5.6 and 0.5.7 just broke
node-canvas.  Entirely.  And the maintainer didn't release another 0.5.x
to fix it. >_<

This took a long time to track down, so let's save everyone (including
ourselves) the trouble.  Pin a more specific version, since we now have
no confidence in jsdom's version numbers.

[0]: https://github.com/tmpvar/jsdom/commit/6ed9c6bc0d882c034e8710dc23e6d01e35f52255