Remove Rakefile in favor of npm scripts

Remove Backbone's last ruby dep in favor of going all npm. Backbone's Rakefile is a lot less logic-heavy since #1685, and these days it's merely a wrapper around a few system commands.

Instead of `rake lint`, you'd run `npm run lint`, etc. If things get more complicated (need to pass options to lint, create a watch task, etc.), it's easy enough to switch to Grunt, npm hooks, or npm tasks later.