Adding express to dev dependencies

This is fixed fully in the 1.0 branch by untangling the express/lusca/kraken dependency, but in the meantime this resolves the following error when running `npm install` for kraken:

```
npm ERR! peerinvalid The package express does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer lusca@0.1.1 wants express@*
```

Express is listed as a peerDependency, but is not installed directly when testing locally. Adding it to a devDependency solves this.