Change outputTrackerFile path

[rvlb]

Closes #80

This change makes the package behavior closer to what was on v0.4.3.

On 0.4.3, if we did this on webpack config file:

output: {
    path: path.resolve('./assets/bundles/'),
    filename: "[name]-[hash].js",
    chunkFilename: "[name]-[hash].js"
},
plugins: [
    new BundleTracker({filename: './webpack-stats.json'}),
],

The stats file is outputted at the same level of the config file (generally the frontend root).

The same configuration on 1.0.0-alpha.1 has th stats file is outputted as ./assets/bundles/webpack-stats.json, together with the other assets files. It's possible to manipulate the output by appending leading ../ to the filename or by using the path parameter in the BundleTracker configuration, however this can get rather confusing.

Tested on https://github.com/django-webpack/django-webpack-loader/pull/271