Drop support for anything before macOS 10.13

[karbassi]

Apple stopped supporting 10.12 and earlier.

Should we stop supporting anything less than macOS 10.13?

This would allow us to write cleaner code since there has been a lot of new code features in 10.13 and above.

I'd like to hear everyone's views.

[robinstewart]

This makes sense to me. We can link to a build of version 1.5 for anyone who needs compatibility going further back.

[karbassi]

I agree. I think version 1.5 will be the last supported release of macOS 10.13.