Fix for latest mocha version

Mocha is now running on 2.x.x, and mocha-mongoose is not working anymore:

```
npm ERR! code EPEERINVALID

npm ERR! peerinvalid The package mocha does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer mocha-mongoose@1.0.2 wants mocha@~1
```