README.md: Change macOS instruction to LLVM 7
README.md: Change macOS instruction to LLVM 7

cquinn/ponyc,dipinhora/ponyc,cquinn/ponyc,jemc/ponyc,ponylang/ponyc,ponylang/ponyc,kulibali/ponyc,ponylang/ponyc,sgebbie/ponyc,dipinhora/ponyc,cquinn/ponyc,cquinn/ponyc,sgebbie/ponyc,sgebbie/ponyc,dipinhora/ponyc,sgebbie/ponyc,jemc/ponyc,kulibali/ponyc,kulibali/ponyc,kulibali/ponyc,jemc/ponyc,sgebbie/ponyc