Update package.json: modify postinstall script

This will fix issue for windows paths. Node-scripts are adding ./node_modules/.bin to the path by default.
Ok, actually, that was an issue with my bower using the different registry. But still, my bower installed 1.2.8 fails with 'Arguments to path.join must be strings', which means bower version in package.json should be bumped (known issue that came up last week, I believe). See https://github.com/bower/bower/issues/1404.