the lastest version numbers have gone back in time
technically, the latest version (v2.0.0-alpha.7) is a lower version than v2.0.0 from March.. in npm v3.10, this command installs v2.0.0:

```
$ npm i koa@^2.0.0-alpha
```

Only a mild annoyance, but `"koa": "^2.0.0-alpha"` in my package.json doesn't install the latest release as desired. 
