Xcode 6.1 support

~~The unit tests don't currently build, because Specta/Expecta haven't yet been updated for Xcode 6.~~
It's not to do with Swift at all, actually:

```
ld: embedded dylibs/frameworks are only supported on iOS 8.0 and later
```

This can be worked around by developers targeting iOS 7, but this just seems simpler and less error-prone.