Allow to open the simulator with a non-retina iPad in iOS7

With the current implementation the following command always opens a retina iPad simulation:
```bash
rake device_family=ipad 
``` 

With this change it switches back to:
```bash
rake device_family=ipad  # non-retina ✝
rake device_family=ipad retina=true # retina
``` 

✝ *Apparently the Simulator opens with the last used setting when no string is given. Switching to non-retina and running the rake command works*