Update backbone.layoutmanager to 0.8.7

Source: [Official repo, latest release](https://github.com/tbranyen/backbone.layoutmanager/blob/ce125ce25b2614b52a13dd61a72fd6b9fabcccc0/backbone.layoutmanager.js)

Minified with http://marijnhaverbeke.nl/uglifyjs

Updated package.json to match the latest version/deps/devdeps.