Fixed the zf2skeleton composer command

I fixed the composer command to install the zf2skeleton application:
```bash
php composer.phar create-project -sdev --repository-url="https://packages.zendframework.com" zendframework/skeleton-application path/to/install
```
@Ocramius it works also without it but @weierophinney thinks this should be the "official" way to install ZF2.