Upgrade micropromise to ~0.4.0

With micropromises ~0.3.0 I get an error when running the suite:

```
/home/raivo/Downloads/promises-benchmark/node_modules/micropromise/index.js:215
        if(!this._state){
                ^
TypeError: Cannot read property '_state' of undefined
```

After upgrading to ~0.4.0 (0.4.6 to be exact), this error went away.