Builds fine on macOS with rustc 1.13

[tspiteri]

The README states that rustc 1.20 is required on macOS, but it's building fine for me using 1.13. I think this is because libc 0.2.48 was not building on 1.13, but libc 0.2.49 has fixed that, presumably thanks to rust-lang/libc#1247.

[soc]

Ah, nice! I will re-test this and update the description. Thanks for letting me know, Trevor!

[soc]

Hi Trevor, I fixed this in 7341389. Thanks for the hint!