Add support for mongoose 4.x

[aaronroberson]

I'm not sure if any code updates are required to support 4.x but when I try to do an npm install in an application that has 4.x installed I get the following error:

npm ERR! node v0.12.4
npm ERR! npm  v2.10.1
npm ERR! code EPEERINVALID

npm ERR! peerinvalid The package mongoose does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer mongoosefiller@0.1.1 wants mongoose@3.x

If lucky, updating the version in package.json may be all that is required.