Update dependencies (including node-canvas)

This updates all our dependencies to relatively recent versions that will play nicely with newer version of NodeJS, Heroku, etc.

Tested both on Mac OS X 10.8 and Heroku (via https://github.com/mojodna/heroku-buildpack-nodejs). Seems to work great.

@bensheldon or @yuletide, if you don't mind making sure I haven't changed anything that seems ridiculous to you, I'll push this.