Trigger a memory barrier when using 'will'

This won't magically make tests thread-safe, but it should reduce some races when the `will` matcher is waiting on a condition that will be fulfilled by another thread – for example:

```objc
__block BOOL done = NO;
dispatch_async(someQueue, ^{
    done = YES;
});

expect(done).will.beTruthy();
```

Similar in concept to petejkim/specta#18.