Respect outputPaths in ember-cli-build

I recently implemented this addon and noticed that the index.json output does not respect the `outputPaths` config setting in `ember-cli-build.js`. Here is a sample of my configuration and the resulting index.json output from `ember deploy production`:

```
// ember-cli-build.js
const isProduction = ( process.env.EMBER_ENV || 'development' ) === 'production';
const assetPath = isProduction ? '' : '/assets';

module.exports = function(defaults) {
var app = new EmberApp(defaults, {
outputPaths: {
app: {
html: 'index.html',
css: {
'app': assetPath + '/app.css'
},
js: assetPath + '/app.js'
},
vendor: {
css: assetPath + '/vendor.css',
js: assetPath + '/vendor.js'
}
}
});
}
```

```
// config/deploy.js
var ENV = {
build: {},
'json-config': {
jsonBlueprint(context, pluginHelper) {
var jsonBlueprint = pluginHelper.readConfigDefault('jsonBlueprint');
jsonBlueprint.script.includeContent = true;
return jsonBlueprint;
}
}
};
```

```
// index.json
{
"base": [],
"meta": [],
"link": [{
"rel": "stylesheet",
"href": "assets/vendor-13455b8fd285c046141dd54ae73890f8.css"
}, {
"rel": "stylesheet",
"href": "assets/app-efe517a8fe179e4392e64c2a9ce8fe05.css"
}],
"script": [{
"src": "assets/vendor-127d2a9b59896b5ddff4de18193179cb.js"
}, {
"src": "assets/app-774827c1c48acfbdb034cbc16d3f5fb7.js"
}]
}
```