Swift 5.1 support.

**Describe the solution you'd like**
Supporting Swift 5.1 so that module stability can be achieved.

**Additional context**
The current MagazineLayout target is still using Swift 4. This means every time Xcode updates with a newer swift version, we'll have to rebuild MagazineLayout. Updating to Swift 5.1 will make use of the module stability to allow us rebuild only when we really want to.