How to configure with Google maps ?

Getting error when I try to configure with google maps.
**Configuration:**
```
var map = eon.map({
      pubnub: pn,
      provider: 'Google',  // tried with 'google' and 'Google' but didn't worked.
      id: 'tracking-map',
      googleKey: 'map_key',  //google map key.
      channels: ['channel']
    });
```
`Error: Please supply a Mapbox Token:`