[WIP] Started upgrading Express version in tests.

There is a failing test and I don't know why:

```
  1) request(url) .end(cb) should set `this` to the test object when calling cb:
     Uncaught TypeError: Cannot call method 'handle' of undefined
      at Function.app.handle (/Users/callumacrae/tmp/supertest/node_modules/express/lib/application.js:119:16)
      at Server.app (/Users/callumacrae/tmp/supertest/node_modules/express/lib/express.js:28:9)
      at Server.EventEmitter.emit (events.js:98:17)
      at HTTPParser.parser.onIncoming (http.js:2108:12)
      at HTTPParser.parserOnHeadersComplete [as onHeadersComplete] (http.js:121:23)
      at Socket.socket.ondata (http.js:1966:22)
      at TCP.onread (net.js:525:27)
```