do not output the xcopy summary

Since ```xcopy``` is used on Windows, every time I do an install/update, I get an output like this:

```
Loading composer repositories with package information
Installing dependencies
  - Installing jasny/db-mysql (v1.1.1)
    Loading from cache
33 Datei(en) kopiert
23 Datei(en) kopiert
2 Datei(en) kopiert
2 Datei(en) kopiert
```

This change makes the process component capture the output. It's required because ```xcopy``` always prints its summary, even with ```/Q``` (Windows 7 SP1).