fix for 'Playback not found: touch_done_ios7_iphone.base64' when testing on iOS 5 + 6 and 'OS=ios7'

**NB: this does _not_ fix playback on iOS 7.**

**problem description**

in order to test _at all_ on iOS 7, you need to set `OS=ios7`.  

when you are testing against iOS 5 or 6 targets, you should not have to change the `OS` variable.  this is the way it has worked for iOS 5 and 6 (and maybe 4).  

before this fix, `load_playback_data` attempted to load the wrong playback file.

for example, when running against a iOS 6 target with `OS=ios7` the `keyboard_enter_text "some text"` fails with this message:

```RuntimeError: Playback not found: touch_done_ios7_iphone.base64```

`load_playback_data` now tries to load an iOS 7 playback file (there are none), then an iOS 6 file, and finally falls back to an iOS 5 file.