Update README's OSX: ??? line

Hi Martin,

I want to report that I am successfully using ring-span-lite on OSX 10.14 with this clang version:

```
clang version 6.0.1 (tags/RELEASE_601/final)
Target: x86_64-apple-darwin18.0.0
Thread model: posix
InstalledDir: /usr/local/opt/llvm/bin
```