Change express dependency to 4.x

I'm using express 4.2 in my project. When I try to install Baucis I get the following error
```
npm ERR! peerinvalid The package express does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer baucis@1.0.0-candidate.7 wants express@~4.1.1
```
According to http://blog.nodejs.org/2013/02/07/peer-dependencies/ you can change the Peer Dependencies entity to `"express": "4.x"` and it will work with every version of Express 4.