Fix compatibility with Babel 6

Hello,

Thank you for this awesome project !

I just tried to import react-resolver on one of my new project, and I found out that your babel configuration is not compatible with the new Babel versions.

Your .babelrc contain the following:
```
{
"stage": 0
}
```
Which is not valid anymore.

This should be replaced by the following:
```
{
"presets": ["stage-0"]
}
```

Thank you

----

Edit:
The error displayed when compiling with babel 6 is the following:

> ERROR in ./~/react-resolver/dist/index.js
> Module build failed: ReferenceError: [BABEL] node_modules\react-resolver\dist\index.js: Using removed Babel 5 option: node_modules\react-resolver\.babelrc.stage - Check out the corresponding stage-x presets http://babeljs.io/docs/plugins/#presets
> at Logger.error (node_modules\babel-core\lib\transformation\file\logger.js:41:11)
> at OptionManager.mergeOptions (node_modules\babel-core\lib\transformation\file\options\option-manager.js:220:20)
> at OptionManager.init (node_modules\babel-core\lib\transformation\file\options\option-manager.js:368:12)
> at File.initOptions (node_modules\babel-core\lib\transformation\file\index.js:212:65)
> at new File (node_modules\babel-core\lib\transformation\file\index.js:135:24)
> at Pipeline.transform (node_modules\babel-core\lib\transformation\pipeline.js:46:16)
> at transpile (node_modules\babel-loader\lib\index.js:46:20)
> at Object.module.exports (node_modules\babel-loader\lib\index.js:163:20)

L’impostazione di compatibilità si trova in .babelrc; inizia controllando la configurazione attuale di Babel e riproducendo l’errore di compilazione segnalato di Babel 6. Sostituisci l’opzione obsoleta con la configurazione dei preset di Babel 6 descritta nell’issue, quindi verifica che il progetto venga compilato correttamente con Babel 6.

Scritto dal modello di indicizzazione a partire dal testo della issue.