Fix for ability to use priority calculations inside KeppAt macros

Small fix that allow to calculate priorities inside keepAr macros
```
    self.logoImageView.keepTopInset.equal = 70 keepAt(KeepPriorityHigh - 1);
```
From time to time I'm using these kind of constructions, when priority is specified by constant, and some of them just +1/-1 from the original one.:)